So I thought I would try and increase my cadence, and "cadence increasing running plan" didn't throw anything out. I also knew from experience that if I just did a 5K unstructured (i.e. trying harder) I would fail.
I decided to run 1st, 3rd and 5th K at between 160 and 170 regardless of pace or HR and run the 2nd and 4th Ks as recovery, regardless of cadence or trying to get my HR down.
Very pleased with the result - you can clearly see where my cadence increased and there are a few download trajectories of my HR on the even Ks - not as much as I had hoped for but that will improve with time.
It was also encouraging to note that my 'recovery K' cadence was a teeny bit higher than normal.
It was interesting how much extra effort that extra cadence cost - perceived and HR. I did speed up unintentionally during the increased cadence. I can't run a 5K at that increased cadence yet, but within a month - almost certainly.
Next run will hopefully be a long 7.5K 'aerobic' HR run where I am not going to worry too much about my cadence.
I plan to put this workout at least once a week.
Anyway, off to enjoy my post-run peanut butter on toast