I wonder if it's a Leafcutter Bee? I know they do it to Rose Leaves......
Nesting female bees cut out immediately obvious elliptical shapes from the edges of a leaf to make their cells for laying eggs. Since one female might need 20 or so cells, that's a lot of leaf cutting, particularly when the bee keeps returning to the same plant. :o)))

Leaf cutter bees only take from the edges of leaves though, and as Ladyessex says they make very neat rounded holes. Could it be caterpillars? the little green ones can be very hard to see.
(I was once lucky enough to actually see a leafcutter bee flying off with a piece of leaf!!!)
Have you looked very carefully under the leaves? (Especially before you brew lemon balm tea with them...)

Thank you both for your advice, it doesn't seem to have caterpillars on it, possible snails? there do seem to be a few snails hanging around! Very odd! (the leaves, not me!)

Thank you again.

Veryy possibly snails then - they can be a menace.