Thanks for that Grahame and I'll certainly be interested in the before and after shot

Your exif is along the lines I was working with in Menorca, 25 secs keeps us in line with the "500 rule" on a FF camera and adjusting the ISO as necessary.

With the infinity mark showing about 1 cm length on the lens I have put a strip of masking tape across this area (but not blocking movement) and put 5 lines at approx 2mm intervals. If I have then ascertained that infinity is at point 3 for example I can check this hasn't moved in between shots.