I have done a bit more investigative work on my baxi bahama 100 boiler and found that there are a number of subtle things that occur
-Some times the pump runs but the fan does not start (about 1 in 15 times)
-If the above happens or if the boiler starts and cuts out it must be reset before you can try again.
-After changing all electrodes and cables the boiler ran for about 1 minute before cutting out and then went back cutting out again.
-Some times when the boiler fires up there is a soft clicking noise after about 5 seconds, if this occurs then the boiler remains alight for longer(about 8 seconds in total).
-The fan was making a bad noise and the baxi engineer from heatteam (arghh!!!) snapped of the impeller (among other things) saying he didn't think it was important. I have changed the fan ( It is quiter but still cuts out)
Almost everyhing on the boiler has been changed so it is pretty much a new boiler. (fan, pump, flow switches, diverter valve,wiring lumb,gas valve,ignition unit, electrodes and leads) In all fairness the boiler is 6 years old and those parts would need replacing.
Everything now indicates the gas valve or cvi unit however they were replaced only a couple of months ago, also I wonder if the fact that the pump runs sometimes and the boiler does not start could be related to it cutting out when it does start. Plus after changing the electrodes the boiler ran for a minute or 2 so therefore that rules out flame sensor.
Pressure was a bit up and down at first but I drained the system and bled everyhing and now that seems ok.
Alex
p.s. Initially the problem with the boiler was that it would not start everytime. sometimes as mentioned above the pump would run but nothing else. The system would lock out and need to be switched off and on a couple of times to reset it. This happened in the mornings quite often however once it started it was usually ok