Bloody hell Richard, you frightened the hell out of me...
For correctness, I've measured the wobble. Got 3mm right and 3.5 left so seems to be less worse than first thought.
Strange how the eye can sometimes cheat on you. It really looked like 5mm of movement.
Also checked the up/down play again. There is none in whatever crank position.
Started working on the bike right after I bought it, just ran it up and down the drive a couple of times before dismantling. Can't remember to have heard any knock. In fact, found it was running sweeter than the SA.
Thanks anyway for the drawing.