Has any one any ideas what I could plant under a tree

I'm assuming its pretty dry there too? You'll probably have to water occasionally when the weather's very dry if you want to be successful in growing stuff underneath. Have a look at these shrubs; Mahonia aquifolium, Ruscus, Sarcoccoca species. Hypericum calcycinum will tolerate dry shade, but its pretty boring and the flowers are few in shade. There are a few perennials, but I'll get back to you on those if no one else suggests anything.

There are some lovely Ajuga's that have variegated leaves in shades of pink too. they do very well under my old beech tree. [have a look at my plant list to get some ideas] You also get lovely blue flowers. then of course you have a range of bulbs you could use. at this time of year you also have Sternbergia lutea [a yellow flower], autumn crocus and colchicums.
When you plant add lots of compost to help improve the soil quality. and put down a mulch in theautumn .the worms will help to work it in to the soil.

Lamium galeobdolon is another low growing ground cover - yellow flowers and variagated foliage.

yes i have this too :o) as well as a pretty saller purple/pink flowered one.

My Great Aunt Felicity, planted her husband under the walnut tree at the end of the garden, when he passed on peacefully.
Alas, within 5 years, she not only lost her marbles, but also the memory of where she had laid him to rest. She eventually passed on too, and was laid to rest, alas alone.
Fortuitously, the "hurricane" of the late 1980's, brought dear Graham to the surface, when the walnut tree collapsed, and we were able to join them together in peace once more.
The following year, the whole area, where once the walnut tree had graced the land, was now resplendent with Wood Anemone and Yellow Celandine, yet not one had been planted by ourselves.
Nature, I thought, does seem at times, to have all the answers, and was a wonderful way to help us heal our grief.
Bye the way, the walnut tree was rooted into the graveyard that abuted the rear of her garden (smile).

Some of the hardy geraniums , the macrorrhizum group especially, would grow there.

Lesser Periwinkle (Vinca minor) with varigated silver leaves is useful. as it gives colour after the Spring when most of the woodlanders come into flower. Wild Garlic(Ramsons) and Wood Sorrel.are two of my favourites. As Burgundy says Nature can make a lovely display. with Bluebells as well, they\grow through the Vinca.