Thanks to all of you for your swift replies. Its been very informative, as always.
FlightDetente:
Am I right then in thinking that if both aircraft are TCAS equipped, then they do communicate with each other to ensure that the resolution is correct (the obvious failure mode if they don't being that one of them has incorrect altitude information)?
HS125:
A bit more information - we were all on the same frequency, and there was other traffic and we were all being passed information about each other.
One was far enough away and climbing out so I doubt that he could have had a nuisance RA (and he said he had us on TCAS, and we were visual with him); but there was inbound traffic descending above me and as I was crossing the extended centreline that could well have been the potential for an unwanted RA. I suspect that being told to 'Climb Climb' when trying to descend on the ILS makes for a few curses at the technology - do you have to react to that RA if you are VMC and have the "conflicting" traffic sighted? What if you are IMC in this instance?
Its a zone I regularly transit but this has been the first time I'd been asked to squawk mode A only - I guessed the reasoning was to stop nuisance TCAS alerts, but then I'm surprised I'd not been asked this before.
Thanks for the kind words re: using the mode C - I am genuinely alarmed by the attitude of a lot of GA pilots who take the view that if they think they have infringed airspace, the correct reaction is to turn off the transponder and try to hide. I learned to fly in Australia and the attitude there was "if you think you're in the middle of somewhere you shouldn't be, make sure you are squawking mode C and contact them ASAP!" which may make for red faces and slightly stern phone calls but is unlikely to result in a mid-air.
Shytorque:
In general, I'm happy to have mode C on all the time - in fact, I'd be surprised that I'd be let into a busy class D without it. As I fly from a field directly below the said busy class D (I'm sure you can work out both the field and the CTA now!) we do keep transponders off in the circuit generally - to avoid nuisance TAs/RAs to the larger traffic departing our southerly neighbour as it turns downwind above us.
Again, thanks very much for taking the time to reply.
Paul.