Annoyingly, the ball joint - although knackered - didn't fix the knock
Between Christmas/New Year, I took the car back and put it on the ramp, wheels free and gave it a really good going over with big pry bars, Lisa working the steering etc.
Found lots more things on the car I didn't like, but nothing concrete for the knocking sound.
Driving it, it had sounded to me exactly like an ARB drop link (but felt bad enough to be a subframe mount), but I couldn't get any movement on them at all. The only think I noticed was that when the steering was operated, one side ARB behaved differently to the other (twisted in a different way) so I earmarked that for a change.
Found lots of other things I didn't like while I was under the car though;

Yes, that's a crack in the steering rack casting - which is sandwiched between the subframe and chassis (now idea how on earth that has happened, but there will be more on the saga involved in fixing that, later...)
More s**t from the £400+ stainless exhaust fitting;

Downpipe-gearbox bolt left out, causing the bracket to rub

Looks like it doesn't line up quite right, so they obviously thought they couldn't be bothered...
Also not impressed with the quality of a couple of the bends (the 'kinks' in it);
And the welds haven't exactly been dressed/cleaned up!;

I intend to take it back to them and see what they have to say (when the car is properly fit to drive again...)
Wiring loom not secured where it runs under the engine (previously over-tightened and split the plastic, but someone also obviously can't be bothered to even refit another bolt!);

This will be the guys that did the cambelt last year...
I sorted that out by custom grinding a washer to fit in the narrow gap but with enough width to support the cable loom.
Notice the sump plug drips slightly - yes, I think that's been over-tightened at some point in its life and the thread is weak...
I could go on, but that will do for now... Basically although I have receipts for everything done on this car, at various places, it's the usual story - 90% of mechanics out there are lazy-arse bodgers these days, so it's no surprise I don't trust them...