FIXED
The cable snapped just behind where it fixes to the handle. Sean thanks for digging up the Internet items, bit of a wide price range $68 - $195 !! My worry was not getting them in time from the US as I have a hillclimb in 2 weeks. So did a bit of lateral thinking and went to my local classic MG parts store with my bit of snapped cable in my grubby mitt. Turns out it's much the same length as one for a MGB, so took one along with a cable clamp to secure it onto the lock mechanism. Total outlay £ 10.82.
You need to take the bracket off the old cable outer where it is screwed onto the lock bracket on the bulkhead. There are two tangs bent over the original cable, release these and fashion the outer one to clamp the metal end of the MG cable and the inner one to act as a stop to make sure the outer doesn't slip forward. At the handle end simply unbolt the Datsun cable and insert the MG cable, thread it throughout the bulkhead into the engine bay. Then using the little clamp you bought connect the wire cable to the lock mech and bolt it back into place.
Hope that helps any other souls stuck with the same predicament