Hi Eran and Bruno,
Thank you for your responses which although quite different from one another appear to both be quite promising - I feel confident I can use both your ideas going forward.
I was impressed by the simplicity of the idea of "breath in, breath out, focus on your feet ... and then try to catch up with the breath on the next in-breath" - this was something I was able to put into my next sit with a small amount of success.
I found your rather penetrating observations on interference etc to be very valuable, Bruno. I noticed that I was still getting frustrated due to the fact I am still essentially controlling almost every breath (except when I remembered to move my observation away to my feet!) but this was alleviated somewhat by the knowledge that this does happen but that it will eventually get better.
The reason I am interested in just observing breath as concentration meditation is as a stepping stone towards Vipassana (according to my understanding of what I think Goenka teaches). I figure that if I can sort my breathing out then I won't have those frustrations nagging me in the background as I try to observe all sensations in my body - The aim is to learn to walk before I try to run.
This of course ignores the fact that the interference is prevalent at all levels!

You have both given me a huge amount to think about and work with. Thanks again.
Colin.