Many thanks, David. Most helpful.
Two further points. When I saw Jupiter the other day, using the same low magnification, I saw it as a small disc, too small for any surface detail, with its moons around it. The whole group was in the centre of the field of view, with plenty of space around it. Last night the white disc filled the whole of the field of view.
Secondly, referring to heat currents. Last night I was testing another telesceope, with unsatisfactory results. I finally gave up, and, hoping to get some observing time I got out the Sktwatcher. My telescopes live in the garage, which is completely dry, and now I think of it, is probably warmer than the outside temperature, tho’ it has no heating. What we saw last night was within less than five minutes after bringing the telescope out, and it had had no time to cool. Could this be the explanation?