Where abouts in the country are you?
The general view is that the shock is not rebuildable
BUT
I've used a guy in Halesowen (west of Brum) who can re-oil and re-pressurise standard supposedly non-rebuildable shocks as well as re-springing etc
Transformed the 3 bikes I've ridden that had it done (ZZR600, VFR750 and thundercat)
Give him a call, Johns his name, and he'll be more than happy to chat to you and give you a price etc (think the last one was approx £90 for respring and re-oil/pressurise)
http://www.revsracing.co.uk/revs/indexfr.htm
Ben
PS may have a second hand White Power one going shortly if you're interested