If the locking bar isn't fully latched that's a very likely cause as the connector won't lock in properly otherwise and you'll only be getting partial contact with some of the terminals.
Spot on buddy. Just to wind this thread up thanks for all the suggestions. The locking bar about a quarter of the way up, probably courtesy of my sons mate's big feet! I must stress to anyone with similar weird problems it was only a tiny fraction out of place, not the half over I at first thought, so there we go problem solved.
I cured the lots of cranking when the starter motor was changed. Now it starts after a turn or so quite happily but then stalls straight away unless throttled up. Might give it a try though, only a few quid for a crank sensor.