For completeness i should finish by saying I resolved my issue - Vaseline in the contacts for the new pedestrian sensors played havoc with the resistance. Cleaned out with a hairdryer and lots of contact cleaner.
I ended up buying an icarsoft LR 2, great piece of kit, but i probably could have got away with the lower model. Diagnostic tools can give you information, but can't always tell you what issues sit behind that information (fault)