White woodworking glue is OK on polystyrene (doesn't dissolve it) So is epoxy, but it's overkill. Both need some method of holding until they cure.
3M and no doubt others do e.g. water/alcohol based contact adhesives that will bond polystyrene foam too. Also it seems in aerosol form, but don't know if it's available over here: http://solutions.3m.com/wps/portal/3M/e ... e#7_0_382I
I've been gluing polystyrene to painted walls (radiator heat relfectors) with bog standard PVA. I once it's gone very slightly tacky I found I didn't need anything to hold it in place and the bond it such that the PS would be destroyed before it came away.
Alternatively I'd try silicon sealant. Sticks anything to anything