The axle has a round female locator on each side which fits a male spigot on the top of the spring. So the position is not adjustable.
So are you sure it hasn't always been like that?
You can see the male spigot on the upper part of the spring in the photos above.
However if the main leaf breaks ahead of the axle, then the axle can move rearwards on that side.
You need to get it inspected, but a slight variation side to side is 'normal'.
Thanks Dave, it was MOT'D this week and nothing found. You are probably right maybe i didn't notice it before but there is quite a difference. Going to get Caine to look at it next weekend.