Worcester Greenstar- no hot water?

Hi, i was just wondering if anyone could help...

For the last few weeks my 4 year old Worcester greenstar 25si has been losing hot water. If i run the shower or hot tap for more than a few minutes it turns cold. In response the reset button on the boiler starts flashing. I reset it but as soon as i turn the hot tap on again it trips out.

I have had a plumber out and we have changed the plate heat exchanger and all the temperature sensors but there is still no improvement. I have already spent £300 on parts and labour and don't seem to be getting anywhere!!

It frustrates me that the diagonistic system on the worcester boiler is so poor as it really doesn't tell you more specifically where the problem is so i am wasting money by blindly guessing what the problem is!!!

Any suggestions would be appreciated.

Thanks
J