Thanks all for your suggestions and comments. I thought I'd post an update
I decided to be strict with myself and fix an absolute waking-up time of 5.30am. This goes for weekdays
and weekends. Most of my friends think this is abnormally early, but then I'm someone who likes having the whole morning with which to do stuff, and I've always had some monastic tendencies
This comes with the consequence that I have to get myself to bed on time. I
still don't know exactly how much sleep my body needs a night (which is ridiculous, really, I'm a fully-grown adult and should have figured this out by now, but better late than never, huh?), so if I fix one end at 5.30am, I can fiddle with the other end to see what works best.
Today is the third day of my new pattern. On Thursday it went well and I was able to function all day pretty well. Friday was not so good: I didn't get to bed at a sensible time, Wed-Thurs I didn't get a lot of sleep (because I kick-started on Thur morning), and so after waking up and turning off my various alarm clocks I promptly went back to sleep...and woke up so late I missed my appointment with my PT
(On the other hand, the scale said 207 yesterday, first time since somewhere in early November, and I had a rollicking time in the evening with a gf at the movies, so yesterday was not a bad day on the whole

)
Today I woke up at 5.30 and am now (nearly two hours later) gradually getting into the swing of chores and the like. I've made my bed so the tendency to duck back under the duvet has been minimised

The plan is to get to sleep at around 10.30 - 11pm tonight; I figure that if I can stick to this for a week then I'll have a better idea of how much sleep I need, and my body can have some kind of stability and constancy in how much rest it's getting.
I do appreciate all your suggestions, especially as regards the melatonin. But before I go to the doc again (she's been seeing an
awful lot of me over the past few months!

) I'd like to try establishing a solid rhythm and see how my body responds to that before reaching for yet another pill to take.
No offence intended to people who for whatever reason take lots of medication, for whatever reason! Everyone's different, and I'd like just to see how much progress I can make by applying some no-nonsense common sense...if I still have problems in a month or so, I'll get my backside to the doc's - and at least then I can say I've done everything I can to solve the problem myself.