Gloucester:
Striking Bikes - Bristol Rd, opposite Toys'R'Us
Cheltenham:
Cheltenham Cycles, Pittville St
Leisure Lakes
Williams Cycles
Also check Noah's Ark, Chalford (Stroud)
I'd probably look at Cheltenham Cycles first. The other 2 Cheltenham shops are quite close. There are also others in Cheltenham, but I've never investigated them.
No idea on servicing - I DIY.