Hi, I woul suggest Viburnun x burkwoodii, Derek.

Exochorda x macrantha 'The Bride' would be my guess

Erm, I reckon its Spiraea vanhouttei...

Dear All - have followed up your kind suggestions by Googling images.
Derek, I can't think my shrub is the Viburnum you suggest because the leaf is quite different.

Bamboo, Again, the leaf shape is not like the Spiraea.

Badfish, Yes, the leaf looks the same and the flowers similar but the positioning of the flowers (mine are clusters) seems different. The pictures of Exochorda x macrantha 'The Bride' seem to suggest they are not clustered.
Perhaps I'll have to go all the way back to Wales to get a few pieces and try again.

There's an interesting notching at the tip of some of the leaves - but I still don't know for sure what it is...

It looks very like a spirea Snowmound that I have. Very, very similar in fact!

I wondered whether it was Spiraea prunifolia simplicata (not flore-pleno), but the pinkish tinge on some of the flowers suggests not...

Is it evergreen or deciduous, Jhdb? It also reminds me of Kalmia latifolia, though the flowers only flatten out like this once they've been open a while... and its an evergreen.

Following Lynn08's suggestion, I Googled again and HEY PRESTO ! Everything fits - leaves, flowers and flower clusters.
I am quite happy for the hitherto un-named shrub to be called Spiraea nipponica Snowmound.
Thank you everybody (and especially Lynn08) for you help
John B