So I spoke to a physio on the phone from Bupa earlier today (one of the perks of the job, may as well use it).
I described the symptoms and he reckons it's the problem mentioned in the post title (I've no reason to doubt him, he made a couple of assertions that tally exactly with what hurts and in what scenario), so he's basically given me the advice of cooling it with ice/cool compression as often as I practically can, and also a list of quad/calf/knee strengthening exercises for me to pick and choose from and do twice a day.
He seems to think that by the weekend I may be able to run again, and has advised I start out by doing short 30 second runs mixed with some walking to ease myself back in to it and make sure my knee will hold up, before getting back on to the c25k programme
Here's hoping I can beat this quickly and get back on it. Very frustrating that as soon as I was thinking "I think I can do this", my knee responded with "what's going on?"