Aye! I wouldn't worry aboot the radiatore at the moment. First of all, work out what's happening to the old donkey.
The beast sounds like it had a new gasket alright.. but it wouldn't need new bolts .. I think stretch bolts are not that common any more (a Rover thing?). The bolts should have just been tightened down in sequence and then re-torqued after 600 miles.
If the head gasket wasn't replaced properly, then it could have failed again prematurely.. but frankly there's not much that you can get wrong when fixing a head gasket. As long as the gasket faces were clean and the head skimmed etc. then it's pretty much a bolt-it-back together job.
The fact that it's overheating again and there's odd bits of rubbish in the header tank and that the matey sold it as a head gasket failure, hints to me that the beast is not suffering from a head gasket problem.. more likely there's somwthing else wrong, such as a cracked cylinder head or other damage. The head gasket may have been changed as a mis-diagnosis of that other problem, or it may have failed and caused the other problem.
Any road up, get the beast compression tested so you have an idea where the grief is. Almost certainly you'll have to take the head off for a peek, so you'll drain the radiator and cooling system then (which is why you don't have to fiddle about with it now).
Depending on the damage, it could be a second-hand motor is the best route.. but whatever you replace, change the water pump itself if that wasn't changed at the same time as the other work. It's probably okay to use the other new-ish belts and stuff.
Also make sure the cambelt tensioner is a new (or recent) one and probably you can re-use the cam-belt (howls of horror!
) if this and the tensioner were both replaced (as they probably ought to have been) when they fixed the head gasket.