There is an Arriva training centre outside Temple Meads station, so is this for ATW or Crosscountry?
Also, do ATW actually run any services through Bristol?
ATW don't have any of their own services going that way, but their Cardiff Canton based Drivers (can't speak for the Guards) do operate a number of services on FGW's behalf to Bristol TM, Bristol Parkway and Westbury.
As of the May 2015 timetable (don't have December's to hand):
Diagram CF340
06.28 Cardiff C - BristolTM 07.18 (1F03)
07.49 BristolTM - Westbury 08.36 (2C08)
08.45 Westbury - BristolTM 09.36 (2E18)
09.41 BristolTM - Btl Pkway 09.51 (2E18, down as non-driving Route Refresh)
10.12 Btl Pkway - BristolTM 10.24 (2Y12)
Rest break BristolTM 10.30-11.00
11.21 BristolTM - Cardiff C 12.17 (2U12)
Diagram CF343
12.30 Cardiff C - BristolTM 13.18 (1F17, travel as passenger)
13.54 BristolTM - Cardiff C 14.53 (1F16)
ECS to Cardiff W and back 14.57-15.15
15.30 Cardiff C - BristolTM 16.18 (1F25)
Rest break BristolTM 16.23-16.53
17.09 BristolTM - Btl Pkway 17.27 (2D24)
17.41 Btl Pkway - BristolTM 17.59 (2C22)
18.21 BristolTM - Cardiff C 19.24 (2U26)
Diagram CF344
(2x ECS moves from Cardiff C - Cardiff W and back) (5F29/5C87)
18.00 Cardiff C - BristolTM 18.52 (2C87)
Rest break BristolTM 19.00-19.30
19.54 BristolTM - Cardiff C 20.46 (1F30)
ECS to Cardiff W and back 20.49-21.16
21.30 Cardiff C - BristolTM 22.20 (2C94)
22.54 BristolTM - Cardiff C 23.56 (1F36)