In reply to wilkesley:
So I went for willesley's g-clamp idea - went to the local huge DIY store but could only find plastic clamps. I was looking at them thinking, hmmm - can they be strong enough? But bought a couple thinking they would come in useful anyway and didn't cost much. Yesterday I took the chain ring off, got some wood blocks, G-clamped it between them and immediately snapped one of the clamps in half getting it moderately tight! Doh.
Plan B involved a hammer and some flat tarmac. That seemed to work quite well, but reattaching the chain ring I could see it was still bent a bit, so tried the adjustable spanner trick - that worked perfectly. Was riding it last night and its was a quiet and purring as an 8 year old mountainbike that is regularly crashed into trees/jumped off from when it goes into ditches/etc. is ever going to be so thanks all!