I think I’d choose a Sorbus. My favourite is Joseph Rock, but Pink Pagoda is lovely or cashmiriana. Or you could go for the native one with red berries. One of the smaller growing Birches would be lovely too. Or a Holly…I love Ilex JC Van Tol because it grows very tall and narrow and isn’t prickly. The red berries are such a great contrast with the glossy green leaves in winter. If you go to Bluebell Arboretum on line they have a tremendous selection to choose from and you can search by the size. I’d go for something about 6-8 m ultimate height and not too dense.

If looking at a Sorbus check out S. aria lutescens. Native, small and very nice.

Mespilus germanica, the common Medlar has so much to offer. Wonderful blossom in the spring and the fruits (the French call them ''dog's bottoms'') can be made into Medlar jelly. The Mulberry, Morus nigra with gorgeous, succulent fruits. Lastly, Amelanchier lamarckii, the snowy mespilus. Fantastic blossom, black berries (much beloved by birds) and green to copper foliage in the Autumn.

Thanks for your suggestions. I had considered sorbus - we have a sapling already that was given to us by a friend so that could be a good option. Although the medlar sounds fun too! I have had a chanticleer pear suggested and a Malus Red Sentinel which I think both grow quite large.The other thing I had in mind was some kind of magnolia. Any thoughts on any of those?
One thing I'm aware of is that we already have quite a lot of white spring blossom eg from 2 mature choisyas, an amelanchier and drifts of snowflakes (which were all in flower together this year), so was a bit concerned about having too much white all at once.

Rather than Magnolia, what about Cornus kousa 'Miss Satomi' or another non-white flowering Cornus variety.

Thanks so much. That's lovely.

Cornus kousa is lovely, but if you’d like a contrast have a look at Cercis canadensis Forest Pansy … small purple flowers followed by gorgeous purple foliage.