Situation much improved, thanks!

Exchanged winter walking boots for a friend's discarded pair of nuptse boots (down filled I think). They're basically quilted cabin booties with waterproof soles, like mittens for your feet.
Also my dad gave me a HUGE (and I mean enormous) quilted synthetic jacket. It's really voluminous and you need to tame it by stuffing it into a strong draw-string tubular sack - where it packs down quite small. (Reminds me of the silk ballgowns from fairy tales that could be fitted into a walnut or hazelnut )

And this is where I think oldpenny 16 has hit the nail on the head. Yesterday I went out with my gloves, expedition weight underwear, hat, fleeces as before. But this time my toes stayed warm...ish, which was great, and the coat came into its own. Just by having it excluding all the draughts underneath me, the difference was sensational. As Katja says, the wheelchair is an uncushioned sling seat (cushions aren't too good as my sciatic nerve is so tender and they can press into my thigh).

OK, it was a gorgeous spring day, and we went right around Regents Park (seeing boars, camels, ostriches, okapi (relation to giraffe with tongue so long the only part of itself it can't reach is the nape of its neck), donkeys and some kind of caribou or wildebeast (they don't put the names of the animals on the outside of the zoo). Even without a lollipop and a balloon I don't think I could have been happier!

BUT, I got to take off my hat and my gloves and unzip the coat. I was lovely and warm, and I know it was all because of the change of footwear and because the thick jacket was keeping my bum and thighs insulated.

MrLapLap s a work right now, but some friends are coming to take me out in a couple of hours, but I'm pretty confident it will be OK from now on, even at dusk.

Thanks for all the suggestions and support. They've been very helpful!