I know a chap who works for Gaz Shocks (local to me). They build shocks / race shocks for various cars.
I told him that you needed a tein coilover rebuilt and they said they may be able to do it for you, but cant say for sure until they've seen the unit.
If you're prepared to send it to them, they said they'd have a look.
Give them a ring on 01268 724585, the guy I spoke to is Mark Gazzard.
All the best.
PS: He said they are just finishing a coilover kit for the 4/5/6 so I told him to get me some piccies and info soon and I'll put it up here.