Problem with Potterton 80e?

Potterton 80e DHW problem. When I turn on hot tap boiler does not start. If I turn tap off & back on about 10 times the boiler will eventually fire. Once this has happened it will work ok for a few hours but then fail again.
When in the failed state the fan turns very slowly - 5 secs per revolution.
The Potterton fault finding guide suggest the problem could be the fan, the modulator board or the sequence control board depending on whether you consider the very slow fan to be a running fan or not. Can anyone help.
Haven't tested the Central Heating side yet - it's taken a while to diagnose the DHW because once it starts to work as above it's back to square one.