Mystery resistance?

Solved it! Not only is there vertical deviance by about 2 mm for 1/3rd of a turn in my chainring, there is also about 2mm lateral deviance for another 1/4 of a turn so whenever I get near proper chain tension, it promptly fucks up for over half a revolution :(

Roll on student loan, I need you now more than ever. BTW in retrospect i've just realised that this has started since I've learnt to skid i.e. been exerting really high forces in various directions on the chainring, thus bending it.

Your chainring shouldnt have bent from a little skidding.

You may like to feel that you are putting your bike through immense inhuman stresses by locking up the wheel a bit. But the force of each little skid is no greater than each little pedal push up a hill. These 'really high forces' are nothing. If you find your chainring bending is means your chainline is waaay out.