Discussion: So

It has just occurred to me that I used to often discover that knee pain was related to back issues.
Yesterday you had some time off the gardening
Today your knees don't seem too bad.

Also wondering about the cycling position, saddle up a bit and back a bit would tend to straighten your knee at point of maximum force.

I'll set up the new bike, once it arrives, as you suggest and test it out. I suspect the hills on the bike caused the problem. I tend to sit down and grind them out - maybe standing is better for knees?
PS my back has had problems for a long time - really stiff in lower joints...

My advice FWIW is to get proper cycling shoes ideally with cleats, etc. When I started doing lots of cycling (in trainers) I ended up with foot problems. Maybe you have already?

I had cycling shoes but they got knackered and E wanted to use my bike so switched to pedals and toe clips - but will get proper kit for the new machine :-)

yeah standing can help, also spinning a lower gear, but getting that knee angle right is key. The simple home setup method is to put your heel on the pedal, barefoot, and have the saddle at a height where your leg locks out straight with a light pressure on the pedal. Once you have shoes, cleats and move to forefoot on the pedal then this should translate to a good knee angle

Good advice, thanks :-)