Simon,
I think the only thing to do now is to pull that motor out and rip it apart to find the problem. It would appear to be a problem other than cam or big ends as you have said - could it be some sort of contact inside (Distributor drive??) ??? An improper sized bearing (mains??) ??? As you said before, and as shown in the photos you sent, the cam wasn't badly worn, and that has now been replaced.
Did you have the crank reground??? - could be an issue here???, and could still be big ends???
I'll keep my brain ticking over for a while longer, and see if it comes up with some other ideas!!!
GUMBALL UK