Spend as little as possible. It's a boat anchor. Put good parts on it and it'll just be a boat anchor with good parts on.
Tyes maybe, they'll have the biggest potential for improving the ride. Honestly, I'm sure you have an emotional attachment to it but I don't think it's worth spending money on. You could easily spend £50 or £75 on new chain, freewheel, cables and blocks only to find that it then needs a new headset and bang goes another twenty notes plus fitting. Or the BB goes pop, another twenty notes plus fitting. Rear wheel (if you can find one to fit) £35 plus fitting. And after all that it wont be worth much more than it is right now.