Ariston 20 MFS Combi - no domestic hot water occasionally

Thanks Chris, in reply to the question "how long did it take" , if you meant the actual job it took me around 6 hours from start to finish but thats including a 1 hour break to pick up my son and his drum kit from a friends house. If you meant from the start of the problem it was about 4 or 5 months! I left it because the boiler was working apart from the leak and didnt want the inconvenience of having no heating in winter if my efforts had failed. Anyway its done now and I have just disconnected the filling loop to see if that cures the pressure rise. It does appear to be dripping so I'll see what happens over the next few days and let you know.

Sorry to take so long to finish this post but I finally got the boiler finished in May. The filling loop was indeed "letting water by" and after replacing it the pressure rise problem was cured. However a slight leak still existed from the top of the secondary heat exchanger. I replaced the auto air release valve at the top and replaced the 4 'O'rings on the white plastic bung in the end of the exchanger. This was all scaled up so I just scraped the scale off with a blade before replacing the seals. Getting the bung back in was difficult though with the new seals on, but with some pursuasion it went back in. Anyway the boiler works as good as new now with no leaks and no need to replace the boiler which is what the engineer advised me at a cost of £1200 and he could do it tommorrow!
Thanks again for all your help.