After finding that the front bush on the rear diff is once again shagged, have found a supplier of polyurethane replacements.
You have to remove the original rubber bush from the aluminium casing (bit of a pig), then repalce it with the poly bush.
Supplier is Floflex Polyurethane Ltd.
Bushes are not listed on web site, so need to enquire.
Will update when I know if it is any good.
These are also worth a look:
Powerflex Uprated Suspension Bushes - car polyurethane suspension bush, bump stops & exhaust mounts. Bushes: wishbone, anti-roll bar mount, track control arm, beam mount, TCA, shocks, dampers, springs