Round leaved mint - Mentha suaveolens
A mint-like plant that is particularly attractive to Syritta pipiens. It also attracts other species of hoverflies together with Blue-bottle flies, some other nectar-loving flies, and of course the Mint Moth likes it too.
It grows similarly to garden mint, so the roots will 'run' a bit but can easily be contained by pulling it up or growing in a large pot.
It likes a sunny position but does not need a particularly fertile soil.
All of the mint family are very attractive to hoverflies with varying flower times - M. spicata is first and Water Mint is one of the later flowering ones. Round-leaved is mid to late in the succession. Horse Mint is a good one too with a long flowering period. (Many thanks to Judy for this further information on mint plants.)