I go to St Eptoe (GS Shop), about 40 mins from Basingstoke - probably the same from Reading. Best value I have found. Has a nice K100 as a free loaner at the moment.
Another vote for Steptoe (GSShop) Serviced my 1150 last week for a grand total of £200 including parts, and gave me lots of free advice on things to keep an eye on. As a new GS owner that was very helpful.