UK Health form S1

I am totally amazed that your mutuality doesn't understand it. I live "in the sticks" (Province of Luxembourg) and my mutuality instantly recognised the form and started the affiliation process.

It may be though that your situation is different. Since you already have a mutuality you must qualify for that affiliation in some way either by being employed (or unemployed) in Belgium or by being a dependent of someone who is. Only in the latter case do you qualify to receive your UK pension (and healthcare support through the S1 form scheme) directly from the UK. If you are or have been employed in Belgium since you left the UK, you claim your UK pension through the Belgian pension scheme and, more importantly, it is the Belgian retirement age (currently 65) and not the UK one that triggers your entitlement to a UK state pension. If this is the case (and the puzzlement of your mutuality suggests it is), your health care cover here won't change. Also if this is the case, Newcastle should have refused your application for your UK pension which suggests that you completed the application form incorrectly. If you did you should take steps to correct the situation very quickly as you have claimed payments that you are not entitled to and so will have rendered yourself liable to prosecution.

I passed the UK Pension age whilst living here. I can tell you how it has affected me - though there is no guarantee that you will be treated identically!
I cannot remember the specific paperwork which I got from the UK but I did whatever I was supposed to do with it.
I draw a UK state old age pension and I think that has some effect on how things work. For example, as a UK state pensioner, I now get the EHIC from UK rather than from my Belgian Mutuelle and I don't have to pay the €50 Flemish Health Care Tax.
Otherwise there has been no particular change in how things work. I still have to pay for any medication - it is not free for pensioners here as it is in the UK.
I have found the people who deal with UK pensions (I think in Newcastle) very helpful when I have needed to contact them.
I'll be happy to help if you have any further questions.

Yes you will still pay the doctor and get a refund from your mutuelle exactly as you have always done. However mutuelles are gradually introducing a scheme for the less wealthy - which seems to include pensioners automatically - whereby you only pay the doctor or dentist the difference between the normal fee and what you would get refunded. The last couple of times that I've been to the doctor I've only paid €4.00. This seems to happen automatically with the mutuelle telling the doctor which of their patients qualify.
I cannot specifically remember how the EHIC card worked but it didn't happen until after I was drawing a UK pension. I don't think that I did anything and I do remember that I had cards from both Belgium and UK which were valid at the same time. My best guess is that I got a letter from the mutuelle and then a card from the UK which was valid well before my Belgian card expired.