Well, I'm back again, boiler has been up & running fine since new PCB, until today that is. Boiler is now, I'd like to say kettling but I don't think its that. It's more like bubbling as though you have put a straw in a glass of water & started blowing into the glass. That's the only way I can describe it. Very weird bubbling sound, but does not lock out, just vibrates really bad until I turn it off. Checked condensate & is ok, not frozen, changed all microbore pipes to 15mm recently, removed & flushed rads, checked flue & noticed that the top half (plastic) has corroded away, not convinced this would affect the boiler this much. Anyone got any ideas?? Cheers peeps