Finally got round to getting this issue sorted. I've been under the car and couldn't find anything obvious amiss re front suspension. My plan of attack was going to be.........fit a new pair pair of banana arms and be done with it. However, I thought I would just get my local mechanic to have a butchers up on the ramp. He located a very small amount of play in the n/s inner tie rod (steering rack end jobbie). I wasn't convinced based on what I have read about tricky banana arms causing 'hard to diagnose' knocks, but hey presto, it was the inner tie rod banging away. I got the garage to replace it (£11 part) and can now enjoy my Z again.
Hope this helps someone who is trying to track down an annoying knock :-)