Loads of searches all saying defender wheel arches wrong place etc, but am looking at a S2a with nothing in front of the seatbox, and my military 110's wings are a bit battered, thinking of replacing them with some military s3 ones in good condition, and wondering if my old ones will fit the S2a

Inner arches on a defender are the problem.
Think They impede the steering box on a series.
Think you can take the inner wings out & swap.
Also measure the wing across the top to make sure the series bonnet will not leave a gap.