Right. Having had a rest and thought about other things, I now realise that for some reason I was thinking the balls would come out in ascending/descending order, and that one ball could only be consecutive to the previous one. That was silly.
But I’m sure it must still hold true because the outcome would be the same if you did line the balls up in ascending order. Except the 1 & 49. But we dont worry about those.
Also, I’ve realised that I may have killed (wounded at least) this thread, and that other people have submitted far more elegant solutions! Never mind.