Well...

I went to see the doc, and he was quite upbeat. He gave my leg a good going over and said that he couldn't feel any tenderness in the muscle, plus I was showing no other signs of properly pulling my hamstring. he was at a loss as to what it could be, but suggested I followed the normal routine for hamstring recovery which is essentially to rest it for a few days until the pain has gone (it had gone even before I went to see my GP) then start exercises that will build up strength in the leg again.

I think that's where I had gone wrong. I'd given the leg plenty of rest and stretched it out, but the original injury had weakened the muscle and I've not done anything about strengthening it since. So, that's what I have to do.

Luckily, my GP said that some gym work would help, so that's not going to stop me from signing up to the gym again as long as I keep and weight work very light with lots of repetition, rather than low repetition and high weights. I can still do the cross-trainer, treadmill and cycle, too. Yay!