But not hostas...

Mine are used to underplant a hydrangea

I was going to suggest planting with vinca. The periwinkle makes a more solid ground perenial cover for the soil when the lily of the valley are dormant and both will spread around with similar vigour.

Ajuga Reptans Black Scallop

I'd go with Ajuga reptans too - there are several varieties such as Rose Glow, the aforementioned Black Scallop, atropurpurea and others. They appreciate dampish soil conditions just like lily of the valley does. For something a bit taller and later flowering, Astilbes are good, but they definitely need soil that doesn't dry out too much too often.

I haven't got space for a hydrangea but the ajuga & a pink periwinkle look to be absolutely perfect, in fact better than what I had in my 'Mind's eye'! Thank you.
I'm hoping the white aquilegia reappears so I'm thinking I should avoid overcrowding? Maybe start the ground cover in containers?

there are many leaf forms of Ajuga and a pink flowered form too. I have mine around Brunnera, and Mitella [a small leafed plant a bit like a Heuchera and good in damp soil].

Yes - my Ajuga are pale green and cream ( and less invasive than some).

The Ajuga will send out runners with plantlets attached - like strawberries do. Here is a picture of mine. This patch was just a couple plants I started in March. It's a vigorous grower & very adaptable. It's sends up lots of blue flower spikes in May - just as the tulips are finishing up.

http://www.growsonyou.com/photo/slideshow/341182-ajuga-reptans-black-scallop/member/bathgate

Better to get the plants in the ground now, but a word about Vinca, since you mention a pink flowered one. It is easy to become confused about this plant from looking on the web, because, in America, Catharansus roseus has the common name of vinca, and a Google search for 'Vinca' will bring it up - its true common name is Madagascar periwinkle, and its not actually Vinca at all, although its in the same family. Vinca major and minor are also known as periwinkles, but Catharansus varieties are not hardy outdoors in the UK, only Vinca minor and major are, and there is no pink flowered version of that, they are blue, so don't get them mixed up if you want a perennial groundcover outdoors. And if you decide to grow either Vinca minor or major, I strongly advise that, every year immediately after flowering, you gather all the stems together, pull them up vertically and chop them off about 1-2 inches from soil level. If you don't, this plant puts out long runners with growth on the ends, leaving lots of bare patches of soil inbetween bits of growth - pruning it in this way creates bushier plants and reduces its highly invasive nature.

Thanks very much all! Plenty of reading for the dark evenings. I'm not in a rush thankfully☺

There is also a form of vinca minor with purple flowers

Thanks Andrew, found 'purpurae' on the T&M site.
Just my luck, they were doing a mixed pack of 3 vinca minor for £6.99 but out of stock...

Hi, that's probably why they're doing it for £6.99, they will no doubt be more expensive when they get them back in stock, [I don't like t&m] Derek.

With you on the T&M thing, Derekm - a lot of others too, from what I gather on here!

As far as a successional plant, you might consider Monarda. I have a bed of Lilly of the Valley which Monarda arise from in good numbers to attract Hummers. But the conditions for both must be met. In my case the bed is in shade in the spring, good for the Lilly and as the arc of the sun changes this area goes to full sun which the Monarda must have to flower well.

If you go anywhere near T&M they will inundate you with messages for life . . . best avoided!

I quite like a new catalogue for pensive moments on the throne Sheila, but you'd think they'd have used more ecologically sound paper?
Monarda would be a bit too invasive for my modest plot LS2 but I like your thinking! Always liked the idea of a chamomile lawn but settled for mixed clover☺