Hi I'm having a problem with my xj600 "91reg" 

I've had to repair some wiring as a mouse has eaten some wires while its been off the road over the past 12 years. I have replaced a few parts on the Starter wiring and all i'm getting is the oil light and nothing else no clicks from the starter relay.

Does anyone have any ideas where else to check or is it the ecu unit that's the problem.

Have you tried to bridge across the terminals from the starter motor to the engine to see if the motor is in fact working? Turn on the ignition, place a screwdriver on the end of the starter nut and touch it to the engine case. You will get sparks but the motor should turn over. Just to see if that might be the problem before replacing more stuff, the bendix on the end of the shaft can stick if not used for a while and it might just be in need of a good soaking with lube. You can also replace the brushes on the motor if they are worn beyond the max wear point. I did mine on the FJ and it worked like new.