Hi Dominic,
Same for me too – haven’t had to refocus my cameras for years.
The lenses I use have a grub-screw on the rotating bezel which allows the focusing ring to be locked in position once focus has been achieved. In the absence of a screw you could use a small pice of masking tape to hold the focussing ring in the same position relative to the lense barrel.
As for how I achieve critical focus, I point the camera near the zenith, choosing a patch of sky that contains a combination of bright and faint stars and adjust the bezel until the faint stars become visible (when you are not in focus, the light from the faint stars is smeared out, rendering them invisible against the background sky).
Agree with Nick – spiders are a problem this time of year, as well as pollen (distinct lack of rain recently to wash the front window clean).
Best regards
William