Thanks, all.
I do think now that it was movement of the tripod because as has been pointed out on the thread:
1. The colour is the same as B Crucis
2. The position is right next to B Crucis so a shudder would show up next to it
3. B Crucis is by far the brightest star in the field so it is the only one with enough photons to show the drama of a shudder.
4. There is a large sweep - the initial "bump" and then a series of reducing squiggles as it settled.
I think the mystery is solved - pity!
Though you might like to have a look at
this thing I saw a while back which proved to be real! Astroron sorted it!
Thanks again for all your wisdom!
Richard