I have lots of small black flies in my patio tubs and the plants look poorly.

Welcome to GOY Minthouse. Sorry I can't answer your question but I am sure there will be lots of people who will help you.

Must be lovely to live right by the sea - even though you are having problems with flies at the moment!

Do you mean blackfly, aphids - these are tiny flies, rather than just small? Buy an insecticide that treats aphids if that's what they are. If they're bigger than a millimetre, then I don't know, but I'd try and insecticide anyway. You can buy sprays ready made up, everyone has trouble with aphids, so the treatments are easy to find