Problem Solved - at last
I removed the fan for the second time and carried out a more thorough clean including unscrewing the short metal pipe that connects the APS to the FAN. Although this wasn't blocked there was some dust and I think this was preventing reliable suction from FAN to APS. Anyway this has solved the problem and all is running fine now.
Summary:
1. I live in very hard water area and this has caused the boiler to cycle too frequently for past few years on hot water only cycles. No doubt this has contributed to a failed gas valve.
2. Boiler is regularly serviced (by BG) but has had no major repairs since first installed in Nov 1999. New PCB fitted in 2004 to cure lockout.
3. Whilst on holiday for two weeks in July 2007 the boiler was swtiched off completely. Bad storms during this time
may have caused dust from fan to enter APS pipe. Gas valve failed during this time too - probably seized up due to lack of use and cooling. No water, spiders or other foreign objects in the tubes.
4. First thing I did was to replace PCB for latest version (PCB 5111603 ) with single multi-coloured LED. This cost me £85 new from ebay. These are relatively easy DIY fit. New version has improved diagnostics which pointed to a gas valve fault. Corgi would have charged me £300 to fit this. In hindsight a new PCB probably wasn't needed but as I want to stay in this house for another 7 years, I think this is a reasonable/acceptable (to me) investment in the boiler and CH system. I fitted new thermister and overheat sensor for £14 - cheap and removes another uncertainty. Again, these were fine, no reason to change. Very easy diy.
5. Local CORGI man replaced gas valve. This is not a DIY job. Total cost £200. Set gas pressure to lower value since Suprima 80 is range ratable. Higher setting caused to frequent cycling and too strong a flame.
6. APS problem came to light. New APS did not solve. Cleaning FAN and pipes and reseating fan to ensure tight fit against back and flue did eventually solve and all now ok.
Conclusion:
I've spend over £350 fixing this thing but have replaced most of the parts that are likely to fail apart from the fan, seals and the ignition electrode. The Surpima 80 is clearly sensitve to fluctuating pressures inside the case and from the flu fan. It's running much quieter now with less whistling and suction noises.
Thanks to baxipoti and others on this forum for advice and assistance.
References:
http://www.hubacontrol.com/en/products/mechanical-pressure-switch/index.php
http://www.interpartspares.co.uk/documents/index.php
