Thanks for taking the time to post. I will arrange someone to take a look at doing this. I wonder if this is a common problem. Dates back almost a year to when I had our 2 boilers replaced and the installer added a 2nd expansion vessel to our system.
Pablo79, did you find a solution to this problem? I have exactly the same issue. We have 4 showers - 2 of them are fine after the initial surge of hot water and 2 cut out because the water is too hot, but they work perfectly after 6 or 7 button pushes to turn them on.