HELP - no hot water

I am having exactly the same problem. I have a Junkers gas boiler, which operates the water in the shower. After about ten minutes the water turns cold. Just paid 100 euro for a visit from the service department to be told there is nothing wrong with the boiler.

When they came to check the boiler, did they also do a test where they check the flow of the evacuation tube the boiler is attached to? Vaillant boilers (in fact all modern boilers) have an in-built safety mechanism that shuts down the boiler automatically if the carbon monoxide gases are not evacuated fast enough (a built-up of those is really dangerous, can even kill you).
We have a Vaillant boiler and a chimney that is not tall enough, so when it's a very windy and/or rainy day the carbon monoxide gas does not go out fast enough and consequently the boiler shuts itself down and no hot water after about ten minutes. The boiler then starts again once it's safe (after about 10-15 minutes).

You open the tap and add water. Basically just put more water into the system.

Following the water pipes that supply the boiler, there should be a pressure gauge. The ideal pressure for any sealed system is 1.5bar (displayed on the pressure gauge) Anywhere between 1 bar and 2.2 bar is fine.
The filling loop will have a silver flexible tube with two water taps on each side of it. When the handles are in the off position they will be across the flow of the pipe. You will need to slowly turn both of them so that they straighten up in line with the pipe. By now you should hear the water flowing through into the boiler.
You need to carefully monitor the gauge as you increase the pressure, do not open the tap full as that will give a surge of water. Do not over pressurise the system!!

our boiler was perfect but as we missed servicing last year we decided to have it done this year, with the same lot. They came on time and after 10 mins and 125 eur all was fine. next morning no heating. They came two days later, all ok, next morning no heating .. this went on for 7 days and in total 4 visits and countless phone calls and aggravation. By the following weekend when i tried to restart it after the same problem again, I had noticed that a wire was dislodged which was the safety circuit breaker to the thermostat, which when it got cold at night, or the slightest vibration, no longer closed the circuit. I am convinced this was a set up as a prelude to an expensive repair - need less to say we'll not be having them back. Sorry this does not answer your question but when I saw the firms name, I just had to get this off my chest