Before you all get into an argument.... problem's solved. Still suspecting a sludged up system, cleaned the mid-position valve (black, dirty) and replaced the head. Voila! boiler fired up - for about a minute then went into sulk mode again (the boiler, not me). The flow was incredibly hot and...
I sort of feel a bit uncomfortable posting this, having just read god knows how many horror stories about this boiler, but if anyone has any suggestions to make I'll remember you for ever!
The boiler's about 4 years old. Had some previous problems with pcb's, but it's been behaving itself for...