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Thank you for your observations. Only thing is there is no trail whatsoever up from the ground or down from the top (should a snail have been dropped onto the greenhouse roof) its just on the middle panes .
I think the trail has been caused by them feeding - they have a radula ( a bit like a rough strip of sandpaper) that they scrape algae off the surface with. This results in the patterns you see.
There is perhaps no trail to the patterns cos the snail/slug wasn't feeding up until you see the pattern, just slithering along.
Lat/Lng: 50.7, -3.4
OS grid ref: SY0597