Awsome, thanks fellas.
I think I may have undone the steering shaft too much at first but not completely, hence only one ball fell out.
The good news is that I wound down the worm enough to see other balls emerge, popped the ball in and screwed the worm back up and it joined the rest of the crew. It was a bit more involved than that and required a bit of jiggling but no big drama. With doubt over how many balls there should be I'm kind of tempted now to see how many balls there really are. But, judging by the bash marks on the four slotted nut on the steering shaft I'm guessing this box has been opened before. To be honest I always thought for a high mileage car the steering was surprisingly fantastic. Clearly it's been replaced or rebuilt at some stage but there's no guarantee that the ball count is as per ex factory. I'm dead certain though that I haven't lost any more.
Also conquered that four slotted nut by placing that rack part back in the box with a couple of bolts to hold it there, put my weight on the box, then large pipe wrench on the nut was hit with a mid sized sledge hammer once only and that was enough. Would've used a vise if I had one, but it's broken from doing something similar a while back.
Oh and got that valve cap out ok in the end. Needed a lot of prodding and levering from all sides but it slid out eventually.