Keith
I have just had time to sit and work through your explanation and yes that fully answers my question and I now understand how to address the problem.
Once again I thank you for taking the time and trouble to set that all out and I walk away a (slightly) wiser man. I am also relieved that my intuitive feel of the situation was correct (at least I got that one correct). I am much happier now that I can put the numbers in place for the problem as well, as it always worries me not knowing how to go about solving a problem.
Thanks again also to everyone else who took the trouble to try to help.
Now back to the long wait to find out how many of the rest of the paper I got right!
Nigel