Mystery solved, many thanks Retlaw. I took a wander up there today and scouted the bunker fully, it's not in as bad a condition as it appears from the outside. I would not encourage anyone to enter the bunker as it is still a dangerous place, but I felt it should be documented before it is allowed to decay further. Here are some of the pictures I took of it for research purposes:
From the south.
From the west showing concrete platform and remains of retaining wall and soak-away pit, possibly the site of fuel tanks for the generators.
From the north.
North-east showing the blast wall and entrance.
The entrance corridor.
Northern ops/bunk room with roof access hatch.
Northern ops/bunk room eastern wall showing floor level duct and stove flu vent.
View from ops/bunk room towards generator room.
Southern wall of generator room showing exhaust vent.
Eastern wall of generator room with main cable duct exiting the bunker.
rectangular field markings 900yrds east of bunker which may or may not indicate the presence of decoy structures.
Well built from good old Accy brick, it'll be there for 100's of years providing the reinforced concrete roof remains stable.
I'm amazed given the scale of this decoy project just how little information is available about it. Presumably this site is just one in a complex of many. If anyone knows of other sites associated with the deception I would be very interested to hear of them, as it would be nice to record some of it before it fades away.
Many thanks to everyone who has commented in the thread

I hope you enjoy the pictures.
Chris.