If you want an aftermarket LSD, then a very sensible choice is the Quaife ATB helical-gear LSD. It doesn't lock up like the plate diffs, and so it won't spit you off the road in extreme conditions.
The unit is available for about 650 quid. If it's any guide, I recently got Billy Bellinger to rebuild my Salisbury axle with a Quaife ATB and 4.1:1 final ratio, with new bearings and everything, and it worked out about 1500 quid. Highly recommended.
