Well, lots of helpful advice here. I hasten to add that I've not been driving it like this: it was this way when I bought it, - unseen, in Lockdown! The first time I've ever bought a vehicle unseen! When I went back and checked I realised I'd not changed the 2 ball joints on the longtitudinal rod, because they looked OK. However I decided to change these anyhow and correct the angles at the idler. While I had the arm off I checked the steering box securing bolts, which proved to be only finger tight, so I think that's a clue! Put it all back together and it feels much better, though I've not yet driven it. I may still have to take Nathan's advice about completely re-building the box, but I need to get the engine running properly first. I still seem to have some sideways play in the steeering column at the wheel. Is that just the top bearing which seems to be sprung from the back of the wheel into the column housing?
I'm with Nathan on historic MOT exemptions. It does seem barmy, how did it arise?