Well, things seemed to have moved on since my last post...
What a load of nonsense from the Council and, apparently, the MOD. The underpass goes from one public road to another, it is probably technically owned by the MOD, but they don't control access at the moment, so it is absurd to 'refuse permission' whatever that means. It is totally outside the wire.
The solution here is simple and inexpensive and could be done in a morning by a pair of workers and an LDV flat bed filled with cones.
Simply cone off the immensely long slip road up the hill such that there is enough room for a pair of bikes to pass. 2.5m should be ample. This would make the (double width slip road) slip road narrow where it departs the A90, but still wide enough for the largest vehicles.
Gonna send a few emails...