I'm sorry, it's not an answer but I have the exact same model with exactly the same symptoms! You described them better than I could. I gave up with it altogether yesterday and switched it off and used electric heaters in whichever room I was using. This morning (with snow on the ground) it appears to be working again so I had a quick shower before challenging it to heat the house. I'm concerned that if I call someone out it will appear to be working normally so they won't be able to fix it but will still have to charge a call out fee.
MJB
February 2009