Thanks for your advice.
I like the 5k’s and look forward to running them faster. My PB on these was set last summer (2014) when I ran 17:10. My average 5k on the 10k programme has been 17:40. I would be happy indeed to run sub 17 minutes!
This morning I ran a 4k time trial as suggested. Here are my results in mins/secs:
Ave HR 178
I felt ok on this run and a little quicker than I expected after 4 weeks off from hard runs.
Please let me know if you need more info and how I proceed to run a faster 5k.