Stick to the standard spring (nos) if you can obtain one, there used to be a member on here who had a supply of them (sorry I haven't read through the thread if this has already been covered) in my experience, any other spring fitted to these old BSAs tends to spoil them, makes them harsh to fire, makes them inconsistent and less accurate, in all, are generally not an improvement on the standard spring without a lot of messing around shortening/removing coils etc!
Just my view! (I have removed a few dreaded 'aftermarket' springs and the like from old rifles and replaced them with standard springs in the past)
ASM