Thanks for the info,
The reality was that I was fortunate enough to get a clearish run from the start and was just got dragged along with crowd.
However after about 6 or 7k's I was really suffering, (my leg's wouldn't respond, and weirdly my shoulder and elbow really started aching (is this normal!)), and it seemed everyone was overtaking me. So I just picked one of the people who had overtaken and tried to keep up with them, and when I couldn't lost sight of them I just picked someone else and tried again.
I imagine my second 5k was a fair bit slower than my first, but knowing that I was competitive makes me think perhaps I should do a few more 10k and perhaps with some specific training.
Once again thanks for your comments
Mark