I'm pretty sure that apart from Scottish Control and Doncaster Radar, Scottish Mil, Waddington and often Coningsby get told when Hibaldstow becomes active. The Mil are certainly not left out of the loop!
Scottish Control will approve (or otherwise) the use of the airway up to FL150 (the airway starts at FL125 if I remember correctly), everyone else then gets told about it by Humberside.
And yes, the parachute aircraft talk to Humberside Radar as soon as they get airborne from Hibaldstow and usually change back to their own frequency somewhere below FL50 if there is no other traffic to affect. The local guy at Hibaldstow doesn't 'control' the aircraft. The aircraft usually tell him what they are doing and he/she will provide the pilots with information.
The aircraft will have two boxes, on one they will listen out to Humberside, on the other they will listen out to their own airfield radio and they will be in contact with both simultaneously.
Yes, there is a formal letter of agreement in place.