Belgian income tax - ahhhhhhhhhh!

Bet on half as a net and you won't be far wrong. Pretty much every benefit is taxable up to and including things like the software on your company laptop, if you use it or not. Bonuses, less than half net.

Gross annual salary: €64,954 (assumes payment based on 13.82/13.95 months)
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Your monthly gross is roughly 4660EUR, which will give a net of about , 2550EUR.
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The normal white collar annual salary is 13,92 times monthly salary, this includes the 13th month, paid usually in December and the double holiday pay usually paid somewhere between June and August. The double holiday pay as well as official annual holidays are accrued during the year preceding the holiday year. Therefore during you first year in work in Belgium you will have only unpaid holidays. The single holiday pay is simply the salary of the accrued paid holidays.
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- 100% public transport costs covered to work: €550 annually (assumes a MTB annual card)
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This is a net reimbursement equal to the annual subscription fee to Brussels public transport (metro, tram and bus). Usually you have to provide your HR a receipt of the purchase of the subscription.
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- Sales commission to be discussed: + 13th month payment + holiday pay (single and double - what does that even mean??), and how are bonuses calculated in Belgium for tax purposes?
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The tax brackets depend on the total annual income. Here is a table of 2014 brackets, the amounts for 2016 are both higher in all levels. In addition to the taxes you pay compulsory social and healthcare contribution of 13,07% (deducted automatically of your monthly salary). Your taxable income is your gross salary minus your social and healthcare contribution
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With your income your bonus will be always taxed as per the highest bracket.
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- Private medical insurance provided
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In Belgium the public healthcare cover is quite good, it covers about 65-80% of medical fees. Hospital fees and dental care however are not included, so private insurance can fill that gap. You should check the insurance cover, it can be for example 100%, i.e. reimbursing the part the public insurance does not pay, or it can give up to 300% cover meaning it covers also the higher fees (higher than the regulated max. Fees) charged by some doctors.
Employer pension contribution of 4%
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Your employer pays a contribution equal to 4% of your monthly gross to private pension plan. Usually you have to pay a small contribution also (automatically deducted from your monthly salary). The catch is you do motley taxes of this amount on the moment it is paid (the future pension will be taxed). 4% to my experience is quite ok amount, though some employer pay more. The pension accrual can in some cases be used as guarantee for loan, for the rest you can in practice only cash it once you retire (very high taxation in case of early cashing).
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- Work laptop supplied
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Well, this would be granted
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- €40 internet at home payment (assumes this is monthly)
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This is monthly net reimbursement. You do not have to justify it in any watts the employer, but if you'd be audited by the tax man you should have proof of subscription.
- €7 lunch voucher per day worked (assumes 40 hours per week)
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Lunch vouchers are paid of worked days. Your employer pays 5,91EUR and the rest is deducted from your salary. As return you get vouchers (nowadays electronic) which you can use as payment in most of supermarkets, restaurants etc.
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- Work mobile phone paid for
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This is taxable advantage, the taxable advantage is usually 12,5EUR
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- 20 days holiday + 12 recuperation days + public holidays
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20 days are the Belgian standard. 12 days are accrual due to prolonged work week (you work 40hours per week, but contractual work week is only 38hours). Be aware that some employers force the dates/some of the dates when these extra 12 days can be taken.
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- €50 representation allowance (what is this, and is in monthly??)
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This is lump sum reimbursement of small costs accrued when performing your job. It's net alliance paid 11 times per year ( assumption is you'r on holiday one month and during holiday can't have work related costs). You do Not have to provide justification for these coast, but some employers consequently limit the type of expenses you can claim to be reimbursed in addition to the representation allowance. 50EUR with your annual salary and assumed position sounds very low. Usually employers prefer paying higher representation allowance, since it has no side cost to employer, but there are no rules on this so this might be just your employers policy.
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If you are moving from UK to Belgium to start the work you should ask for possibility for expat tax status. You should be eligible and it would mean significantly lower tax to you and reduction of social security payments to your employer.
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If you have Belgian contract you will be taxed as Belgian despite your partners tax status as EU officer. However since she does not pay as Belgian you will be seen as single in Belgian taxation (normally couples income is taxed as joined income).

You work out your take home pay and get a number but, unless you imagine that every single thing costs exactly the same in Bxl as it does in London, the figure is not hugely helpful. It doesn't tell you whether you will be better off, more or less able to save, able to enjoy the same standard of living and so on.

I moved from London where I lived and worked in the outermost suburbs. My salary here was more-or-less identical and within a very short time I realised that I was enjoying a standard of living which I could only dream of in London. Principally because accommodation was so much cheaper.
€64,954 per annum is a better salary than most Belgians. I'd guess better than 80% of Belgians. It is a guess though €2,500 a month would generally be seen as very reasonable to manage on.
My son is married with a teenage son and a mortgage. Their gross annual family income is nowhere near €40K but they live reasonable comfortably. Not in the lap of luxury but they have no financial worries; nice house on a good plot of land; run two cars; holiday abroad each year and so on.

The bottom line is that, whatever your actual take home salary is, you will have enough for a very nice lifestyle; even more so with a partner working full-time.

You won't be happy if you do it for money. Do it for love.

Roughly, housing will cost less than half what you'd pay in London, and a season ticket for public transport about a third.

€4600/month is a very decent salary - you can live very comfortably on that, even if tax and social security is close to double what it costs you in the UK.