Thanks Guys, that was very helpful. I also asked this question in the mil forum and the consensus of opinion seems to be that there are a wide variety of traffic alerting systems of which TCAS is one, and that most mil aircraft are not fitted with it. I will continue to squawk 7000 mode S just in case one of the birds coming over my head is fitted with the appropriate system, but a very good lookout is the best defence.
I have spoken to Flight Ops at Odiham whose recommendation was that I contact No 1 AIDU at Northolt to get my HLS marked on the mil charts. As I also get Hercules over the top quite frequently this seems to be a good idea and I will do it today.