Well the verdict:
I have a pre-existing condition (Baker's cysts) but that should have no effect whatsoever, apart from causing them to swell a bit the first few times I go out. That's not news, as that is what happens every time i dont do exercisthen do some.
Running is not ruled out just because my knees dont work properly. The cause of them not working properly is sitting down too long for several years now with minimal exercise. Therefore a gradual increase in exercise should improve things rather than make them deteriorate.
I have some exercises to do to strengthen the thigh muscles as these are the muscles that support the knee joint. Need to do these as often as I remember but at elast once or twice each day. They include lunges, knee bends and leg raises (two different ones).
As long as I start gently, I should have no problems. The programme I have chosen starts with regular walking, then walk/jog, then jog/walk, before jogging full time. It takes about 7-8 months to get to running full time and then it is only for half an hour a time, so should be fine.
All I need now is to get some proper running shoes to limit the banging to my joints as i run.
Finally, the place I have to run is an old railway track that is now a footpath. It is edged with grass (no pavements) so I can run on there.
Sorted; thanks for your input, everyone