Wait till they're going dormant, then split the bulbs - I assume you take it under shelter for the winter, but if they didn't flower this year, feed them now with a balanced liquid fertiliser weekly, while the leaves are still functioning. As soon as the leaves start going over, stop feeding.

Thanks for the advice Bamboo and no, it's always been outside.

Oh right - which variety is it, do you know?

It looks the same as 3d bit's picture the other day. My photo last year August 4th page 10 of my pics but I don't know anything else, sorry.

Looks like H. festalis - these are not hardy,so its usual to lift and store over winter, or take the whole pot into a greenhouse out of the cold. Goes to show how mild the winters have been since 2010 that yours is actually still growing, even if its not flowering.

Yes, I have to agree Bamboo. This year we had a couple of frosts but nothing too bad and the previous couple of years the same. You must be quite similar to me weatherwise living in London. I live at Heathrow which usually comes up as the hottest place when we have a heatwave. I can't take the pot in as it's huge but might think about lifting the bulbs, as you say. Thanks.

Well I'm not far from Heathrow, I'm in West London and yes, its always the hottest bit of London, nightmare!

While tidying the large pot where these are I found a label I had written which said Ismene festalis. An alias perhaps?

A synonym yes - actually, its been renamed, along with a lot of other plants, due to DNA sequencing usually, so now its Hymenocallis, but you'll still find it on Google if you put in Ismene.