Why is my Ferroli Modena 80E combi boiler not heating water for my shower?

I have a Ferroli Modena 80E combi boiler and I am at the stage I feel like throwing it out the window. The water heater will start when I start the shower but after approx 1 mintue it cuts out again and the water is freezing. There are no warning light or anything flashing on the system. It just stops. This boiler worked perfectly until a British Gas representative came out to check it to see if it was worthy of going under their insurance scheme... he worked at it for a long time to see if it was allowed. I paid £17.00 every month (which I paid for 4 months even before their rep finally appeared) for this insurance and when I informed them there was something wrong ever since their Rep was out they told me it wasn't their fault and they could do nothing about it.... I am a single girl with low income and cannot afford to pay for a plumber to fix whatever British Gas have done... can someone help me fix this? All I need to know is how do I get the bolier to stop cutting out for the water in the shower.... Please, please help!!