It is many decades since I last had anything to do with DF so the memory is a bit hazy but HRDF was the term used by the MOD/RAF for its new DF systems bought in the early 80s. The predecessor was termed CADF. The HR is High Resolution. It was higher resolution than its predecessor but still not particularly accurate. NATS bought the same system for its airports around the same time. The same equipment is often termed DRDF outside of the MOD with the D being Digital or Doppler depending upon your preference.
The system supported VHF and UHF, you just slotted in receiver modules for the channels you wanted to cover. The MOD DF network formed the back bone of the Distress and Diversion triangulation system but when located on an airfield also included the local frequencies with displays in the Tower. Many of those airfield locations are no longer RAF airfields, some are no longer airfields at all, but the equipment remains part of the D&D network. Project Marshall was supposed to update and rationalise that equipment but I have no idea how whether it did or still will, indications from Manston are that it hadn't in the original timeframe at least.
Northolt is a slightly special case. Northolt Approach is at Swanwick and the workstation they use has no provision for either a DF display or integration of DF equipment with the radar display because it's a standard NATS Workstation and NATS sees no need for it at approach positions in the London TMA. It remains part of the D&D network though.