K-tec are very good at supplying parts quickly via post, if you go to them to pick up parts they are very friendly and the tall guy behind the counter/ workshop knows GT turbos very well.
I've had my car in there 2 times for work to be done but not again, pm me if you want to know why.
GT Tuning- i have found Roland to be very helpful and knows GT turbos inside out, i used to work with a guy (zeppy) who sold the RAIDER which became B5 GTT to him in the early mid 90's, ok they have moved on to Mini's and TT's but they still have stock of GT Turbo parts and K-tec tune/ mod lots of other cars aswell.
Both of these tuners are a local to me, so i don't want to offend anyone but like GTT BOY says he likes to do his own maintenance/ tuning, i agree, do as much as you can yourself, tuners are 50 quid an hour.