When I did it, I asked Interflight which frequency they would expect me to call them on first as I believe there are several, approach, director etc.
I was inbound to Gatwick all the way from Shoreham. I called the freq Interflight had given me. To be honest the controller was a bit confused at first because I guess most of her traffic is handed on from an approach controller. Anyway after a couple of seconds she must have found my details, 'cos it all turned out ok.
Anyway listen to the ATIS, I'm sure it will say something like report Information "xxxx" and aircraft type on first contact. Then add G-XXXX inbound from Thruxton.... The controller will tell you "radar vectors to the ILS rwy XX.
In true Pprune form, I wait to be corrected!