It's most likely slugs, snails or caterpillars. They do their damage overnight. I live in the US, but some repellent like Sluggo or Bug Getta usually works.

Most often tobacco budworm, around here. They chew at night, and hide incredibly well during the day.

In the UK 'neat' holes might mean round the leaf edges, in a half circle, or it might mean circular holes within the leaf, so a photograph or more description would be useful.

Could be flea beetle ... They make small holes in leaves.

I'd go with flew beetle too. They decimated my Johnsons blue pelargoniums.

Umm..'Johnson's Blue' is a true Geranium, not a Pelargonium. I've never had flea beetles on my Pelargoniums, but that may just be my location and/or climate.

No beetles or any bugs on my Pelargoniums either Tugbrethil, here on Long Island, New York.

But you are both thousands of miles from here. Flea beetles make holes in my Pelargoniums and other plants also. It happens every year. They are difficult to get rid of.

True Hywel! I was thinking it might be a species of flea beetle that we don't have here in the States (yet). Please don't send us any: we have way too many bugs anyway!! :O

lol ... we'll keep them here then :)
Bugs and other things seem to be spreading though. I think they come with imported plants. I saw a Lily beetle for the first time this year.

Our woodlands are being decimated by the Asian Longhorn Beetle and the Emerald Ash Borer.

Vine weevils? Just a thought!