Hi,
I've no experience with the BS7, but I think that these shocks are the same type used on the BS50/60/90/100 range, with different covers and springs. These shocks are all quite easy to take apart.
They have the bottom fixing eye screwed on to the damper rod and fixed with a hidden locknut. Hold the bottom eye of the shock in bench vice and lift the lower chrome shroud by hand (the spring isn't that strong on a '50). You should then see the smalll locknut on top of the alloy casting. Simply loosen the locknut, and then unscrew the bottom eye from teh damper rod and lift off the shrouds and springs. Before reassembly check the damper for any oil leaks, and pump the damper rod a few times with the shock vertical to mke sure it is still damping OK.