An ATZ, whether around a civil or military airfield, goes up to 2,000ft above aerodrome elevation (not agl); a MATZ around a military airfield is up to 3,000ft above aerodrome elevation (once again not agl).
An ATZ is in effect during the hours notified in the AIP, either in the relevant AD section or in the ENR section which contains a list of ATZs for non - licensed airfields and the frequencies for calling them, however there could still be 'out of hours' activity so I would recommend a call on their tower/AFIS/Air Ground frequency.
A MATZ is only in effect when the airfield it surrounds is open for 'operational' flying so it's recommended (ie not compulsory) you make 3 calls on the notified frequency and if no reply, you can assume it is not active however, as you say there may in some cases be recreational flying in progress and if it involves gliding, watch out for the maximum height of winch launching notified on the chart; steel glider cables are very effective at sawing into wings whether metal or wood/fabric.
As Gertrude says, forget RPS and just use the QNH of the LARS unit you are working. If you do use RPS and are flying just below the base of controlled airspace, there is a risk of infringing; I have watched it happen on radar in the days when the RAF didn't use a TMA QNH and several Hercules routed through Midhurst VOR on the Chatham RPS without talking to anyone and infringed controlled airspace causing Gatwick to have to stop departures.