It probably has a bunch of problems Mark, with compression just the first of them. If you take off the rocker cover you can see if it has a stuck valve. Also of course, check the tappet clearances, observe the movements of both valves as you rotate the engine, and watch for the action of the decompressor: the exhaust valve should "bump" open slightly and momentarily in about the middle of the compression stroke. Look whether the head gasket is intact: I've never heard of one blowing on a GXV140 but if the previous tenant took the spanners to it, who knows what might have been affected.
Once you've done that simple stuff, if you haven't found it I think it will be time to take the head off to check for more serious problems like a broken piston or connecting rod.
