Looks like a Chrysanthemum to me.

Yes, but a very straggly one. Cut it right down when it has finished & plant out somewhere. It will grow again in Spring & won't be so leggy.

Thanks for the info.

I've Googled 'leggy chrysanthemums' and it seems they get like this if they're deprived of sun. This one came out of a shady garden, and has been sitting out of sight at the end of mine since.

I've just been preparing a new flowerbed for next year, so will stick in in the ground where it'll get more sun in future.

I have some tall ones that are also small flowered so it may well be the way it grows. It is a healthy green so I don't think it will be smaller even when planted out. it will need staking more than likely.

It could well be the variety 'Ruby Mound' that grows two to three feet tall

I don't know about 'Ruby Mound', but many varieties benefit from a couple of pinchings in spring and early summer.

I gave 'Ruby Mound' a Chelsea chop this year, and it was better for it

I've always whacked mine down to near ground level in January, which is when the new basal shoots come out, here. Probably best done later in colder climates.

I cut mine down either after they have gone over [Dec] or in March as the new basal shoots start growing.