Turns out my knee pain is probably jumper's knee aka patella tendonitis. I'm having it x-rayed on Friday to rule out a chipped patella (ouch!!) and have been give some NSAIDs to reduce the inflammation. And I've been told to see a sports physio ASAP. The great thing about having a GP who runs himself means you're unlikely to be told "Don't run."
Hopefully a bit of TLC will see me back training fully before long :-)