Went ahead any purchased a new auto-bypass valve as the bypass pipe was getting as hot as the C/H Flow pipe right away which could have been caused by a system blockage were it not for the fact that system water flow was fine.
Removed pump head while boiler was drained just for peace of mind and that was clean but again, not surprising as flow was good. Re-Filled and ran - Bypass pipe still warmed but much slower than before so now the boiler wasn't going into shut-down/anti-cycling from flow water re-entering HEX on return side but it still shut down with F.22 fault!
Having a selection of NTC sensors purchased previously I re-visited the flow and return ones to find for some reason during swapping them around I had two non-identical NTC on Flow and Return so I changed all 3 (Flow, Return and DHW) for the 3 cheap ones previously purchased on e-bay which I'd checked and all appeared to be within tolerance on both cold and hot resistance readings.
Also... for anyone else having irrational issues, the NTC sensors on this boiler are single wire back to the PCB with the "earth" return being made via C/H pipes back through the boiler chassis which is a really bad idea when joints are made with o-rings and fittings get corrosion. Only realised this when found youtube vid of a plumber chap finding a bad return pathway was causing intermittent F.24 fault on the same boiler - he fixed it with a wire strap and clamp back to chassis which is next on my list as I still have 60 ohm resistance from flow NTC back to chassis but for the moment I've sprayed contact points with switch cleaner.
All was working well last night and thismorning for the first time in around 18 months the boiler had come on at 07:30 and we were greeted with a warm shop. I even re-connected the actuator to the diverter valve as that was also causing the boiler to F.22 when DHW was drawn and that too has been working perfectly.
Still early days but the theory of trusting that the PCB actually knew something was wrong and was reporting the fault to it's best ability all be-it completely wrong rather than changing the PCB has paid off.
Early days yet but it's looking promising!