Thanks for the help so far.
Had to live with the problem for a while but finally got some spare time a couple of weeks ago and replaced both rack ends and track rod ends and had the tracking done. Fairly straightforward except the new track rod ends came with poor quality nuts and the threads stripped out of both of them before they were anywhere near tight. Reused the old nuts and they tightened up no problem.
There was a definite improvement in the vague, wandering feeling in the steering but occasionally still a bit of vibration. Was hoping it was just slightly out of balance wheels or uneven tyres, but a couple of nights ago driving slowly over a temporary road surface the front end was shaking really badly and there was a loud, fairly slow knocking noise.
Also for a few weeks there's been a horrible rasping noise above 4000 rpm that sounded like the exhaust was blowing - I'm thinking that maybe some excessive engine vibrations have cracked part of the exhaust or the downpipe bracket. There seems to be more dash and trim rattles in the cabin than usual as well.
Had the car up on stands tonight and can't find any play or knocks from the suspension, but if I grab the engine I can shake it a bit from side to side and the top engine mount waggles noticeably - is some movement here normal or does this sound like the mount needs replacing?