Thanks for the comments.
I’ve realised today that my weight does tend to drift from the saddle when I
“launch” to the pedals about 10ft down the hall (it’s a big hall, by the way,
rather than a corridor). I guess I need to concentrate on this to help with
direction.
I thought I’d noticed it was easier in the other hall which has a wood floor
(and also in my wife’s school hall which I’ve used once), so thanks for
pointing out why!
Today’s main lesson (I’m doing 20-30 minutes a day) was the importance of
keeping the pedals “horizontal” (ie, thinking in terms of half or whole turns
rather than continuous pedalling). I felt a great improvement today - now
occasionally having to stop because I’ve reached the other end of the hall (I’d
estimate about 35-40ft) rather than because I’ve lost balance/control. I’d love
to go outdoors, but It’s too wet & cold!