I need to replace a damaged radiator, the old one is very old so I can't get a direct fit replacement.
Now as I'm going to have to connect onto old copper that might be damp or contaminated I wondered if it was ok to make the first if not all the connections with compression fittings.

Aye assuming sizes are checked. Remember to use a goodly amount of PTFE tape on the olives and teh threads.

Yes. Just make sure the pipes are nicely cleaned – use wet & dry plus wire wool – and perfectly round – no dents. The olives will make a perfect seal.

Always best to go to a proper plumbers merchant for your bits, because they can give a bit of advice too. You can get a compact pipe cutter to make things easy.

as above

I've got all the tools, but it's been years. I don't even know which end is fed, but I'll work that out.
Guess is it's the end with the drain off nearby as oppossed to the end with the balance lock valve? or whatever it's called.

Shouldn't be a problem to use whatever you favour. Solder, compression or push fit, they all work. Old copper shouldn't be any harder to work with than new, some deburring tools take 3/4" of old paint off too which makes it easy on old pipework.

The paints not that thick 😉

Use new olives (don't be tempted to reuse the already squashed ones), don't over tighten, and get some Plumbers Putty to make sure you get a decent seal.