They are deciduous, so I'd wait a while until they start to die back. You don't want the crowns to rot, which could happen if the mulch gets really wet. I'd mulch around the plants rather than on top, I think. They're hardy plants, so let's hope they've got their roots established before we get any severe weather..

They should be ok in the ground as they are hardy like spritz says. They are good doers as they say.

That solves my dilemma nicely. I'll mulch the soil around them and leave the crowns exposed. it's been glorious today (saw some wild garlic in flower - they must think it's spring). After the last two years I think we're all expecting the weather to turn freezing cold at any time. i know I am. I've lived in Exeter for 20 years and only known prolonged freezing during those years. It's a relief to know that they are good doers!
Thankyou Spritz and SBG.