Thought I'd read somewhere else that a Meteor spring might be the one, that pretty much confirms things, thanks.
Hopefully I will get around to separating the barrel from the cylinder tomorrow & get a better idea about how to rejoin them without the wobble. Guess if it goes back out of line with the scope rails I'll stick to the open sights on the barrel! Wonder how they make underlevers at the factory; would they join the barrel to the cylinder & then 'bend' the barrel so it shoots in line with the cylinder? It's just I've never noticed one poi drifting left or right with changes in range. Underlevers I've fired all seem to shoot straight.