Aeros is normally manned until 17:30 local but they do close early from time to time if the weather isn't permitting flying training. Suggest you give them a call on the day of your flight to check.
If Aeros is closed, it is possible to get landslide via the fire station gate, although that may be available to persons having a Cardiff airside pass only - I suggest you ask Aeros that when you call.
Cardiff is a GA-friendly airfield so you shouldn't have any problems flying-wise. If coming VFR, expect to be given entry/exit to the zone by one of the published VFR entry/exit routes (see AIP or Pooleys etc) so suggest you keep a copy to hand.