OK, so I did some more testing last night. I opened a brand new project, had a bass line and a kick drum. I set up the PC bus compressor on the bass, sent the kick drum to it and nothing happened, I tried switching the pre/after fader toggle on the send, nothing. Then I switched the mono/stereo switch on the bass track and voila, the compressor started pumping with the kick drum. Once it started working I could then turn the bass track back to mono (it lowered in volume when it was in stereo) and it carried on working. I then tried the same again but used a bus channel and then I even set up some patch points and an Aux track to see if that changed anything but all was still fine.
So I then tried the same on a full project (60+ tracks) and the same again, and it worked straight away (almost like something had cleared in Sonar and it was now fixed) but.........
The only problem I then had was I could not now solo the bass track to actually hear how it was being affected by the kick drum, but I kept soloing and different tracks ( I had a bass part sent to 2 patch points, these to an Aux track and this to a final bus which I sent the kick drum to) and finally again something seemed to clear and it let me solo the bass track and I could hear the pumping effect.
I will carry on trying out a few things and go back to the vocal track that I originally wanted to do and see if that works but it seems to me that somehow all of my routing had got messed up somehow and then in some way it managed to clear itself. Very strange indeed.