It looks to me like bad thrips damage. Systemic insecticide will help with that, though I prefer to apply earthworm castings and/or kelp meal or washed seaweed. That's slower acting, but much better for the bees.

Thank you. I'll look for something to control them with.

Look into the flower closely. If you can see tiny (like 2m long white wriggling things then you have Hemerocallis gall mite. No cure. All you can do is learn to recognise the affected flowers at bud stage and remove them and dispose, not compost.

If the gall mite, systemics won't work, but the organic remedies should help some. Definitely remove the buds as Owdboggy says, too.

Thank you both. I couldn't see anything inside the flowers, but they were all affected so I have chopped it down to the ground. After it regrows I'll try an organic remedy and keep an eye on the buds.

Hemerocallis gall midge. Systemics will work on those, but I am always reluctant to recommend them, for the sake of the bees. According to the RHS, the only organic control is for you and your neighbors to remove affected buds. It seems to be more prevalent on early blooming varieties.

Thanks. Nobody else around here has Hemerocallis, there are no 'gardeners' living here. They all have lawns and a few shrubs.
I've got some more Hemerocallis but they are not infected. They are in a different part of the garden.
I'm hoping that cutting it down to the ground will help. When it grows back I shall keep an eye on the buds.

According to the RHS, it's only found in the flower buds, and is supposed to be done with in mid July. More info here:
http://www.daylilies.org/ahs_dictionary/gallmidge.html

Thanks for your help :)

I get this on early flowering varieties Hywel. the adult flies and lays her eggs May June and is also very small 1-2 mm. as soon as you see a swollen distorted bud snap it off and dispose in the household bin. later buds on the same plants are unaffected. The swollen buds are very wet inside and with 6-10 maggots 1-2mm long inside.

it is another pest that is working its way north. my brother in Sunderland rang me about his just last week. He has never had it before.

Thank you. I am now wondering if that is actually the cause. This is flowering in July so it isn't an early variety. Maybe it's slugs. I'll be moving it to a different place soon.