I've been using DarJac Racing near Colchester for the last couple of years, he's well respected on M5Board.com and owns an e34 M5 & a Skyline himself.
Give him a call and tell him mmm-five (Tony) sent you - although I know he's being kept quite busy, so may not be able to fit you in immediately.
http://darjac-racing.co.uk/ (forgive the website, but it's just a holding page for now).