Hi Rob - sorry to hear you are having trouble.
Are you continuing to stretch when you return to full activity? Are you gradually increasing your mileage over time?
Typically, if the injury is returning you are not addressing the route of the problem. Tight hamstrings (ask any club member to try and touch their toes - a blessed few might get there) are a common complaint in cycling. As you do not fully extend your leg when going through a pedal revolution this tends to lead to a tightening of the hamstrings which, if left un-checked, can cause a whole range of issues from knee trouble to lower back pain (my returning pain in the ass - or backs should i say). Keeping on top of your whole body flexibility can really help to prevent issues arising in the first place or indeed from returning after a period of rest.
Also consider your hydration status as a key element of this recovery. Typically dehydration can exacerbate overuse injuries and reduced range of movement at joints - keeping ontop of your hydrate can really make a difference and is commonly overlooked by many. Adults should aim to try and consume 2-2.5L a day (8-10 glasses). Consider around 20% of your intake comes from food - slightly more if all you eat is watermelon and cucumbers
- I suspect you would also be the first to the top of most climbs - I'm pretty certain that's what Joe's secret is...
Finally, get an experience club member to take a look at your setup - they may be able to advise where the issue is arising from. A very loose rule of thumb - saddle too high = back pain, saddle too low = knee pain. Might be a good place to start.
Should any issues continue then seeing a professional can really help - I don't know if there are any club members who can recommend a cycling savvy local physio?
Hope this is of some help to you,
Cheers