Thanks Martyn,
I didn't think of the density of wood used.
Just looked at the air arms site and it gives a weight difference of 1/2 pound as i expected so if i was buying a new gun and opted for the walnut with one of the expected benefits being a lighter fun i wouldn't be to happy with one that was heavier than the cheaper beech option!
I will probably keep an eye out for a second hand walnut stock but don't think one will turn up in my price range, but you never know, if i get the current stock sorted in the mean time I'll most likely keep it as the air arms gods intended.

Cheers johnj.