Just an addition to what Muppit said...
Surrey Ambulance (strange being as LGW is in Sussex - but due to budgets years ago!) provides Gatwick Solo in a joint funding operation with BAA. Solo is based at LGW and only responds to incidents within the airfield boundary. We operate 0600/2400 7 days a week. This has greatly reduced the AFS turnouts to medical calls.
Most of the ambulance cover/backup is provided by Surrey usally from Gatwick itself, (an ambulance is there 0600/1800 Mon-Fri) or nearby Horley. Sussex do help out but their unfamilarity with the airport often causes problems!
All of the solos are free range trained and also work on the Gatwick and Horley ambulances thus reducing greatly the workload on the police to provide escorts airside and reducing attendance times.
When solo is off watch or committed AFS will still attend medical calls. Their staff are well trained in basic ambulance aid, including defibrillation and pain relief. Unlike some fire services I have dealt with over the years, we have an excellent working relationship with AFS, Airfield Ops, BAA staff and hopefully the tower staff!!