SVE Autocraft in Sailsbury do a pro charger conversion but you are looking at 5k and then you need suspension, brakes, quaife diff etc. Vortech did a kit also but discontinued it because it had a 100% shaft failure rate.
There was a company in the USA called Arizona Dyno Chip selling a single turbo conversion but it was on hold for more development. Have a look at
www.contour.org 
I looked at this with my st24 but just bought an escort cossie in the end coz it worked out far cheaper.