The flowers are white
By Edwardlam
United Kingdom 
I believe it is a simple flower with 4 petals.
Philadelphus does sound right, possibly.
Thanks all, for your trouble.
My own search threw up Deutzia 'Nikko' Gracilis 'Nana'.
I'd be grateful if anyone has a pic of that one.
Thanks again.
29 May, 2011
Answers
Please could we have a picture of the bush and a flower, fairly close up?
29 May, 2011
As Beattie says we will be better able to i.d. with more photos, you can just add them to your question.
29 May, 2011
Could it be Viburnham plicatum?
29 May, 2011
Could be lots of shrubs... flower and photo of actual shrub will save us from playing a guessing game.
29 May, 2011
My first thought was Philadelphus, but if we get a pic of the flower & the shape of the bush we should be able to ID it properly.
29 May, 2011
Just Google that name and you'll get plenty of photos. We can't put up photos in replies. In fact I just Googled and leaf shape isn't right and the flowers have at least 5 petals.
30 May, 2011
P.coronarius has 4 petals, not ALL forms are double. And I wasn't sure about the pattern of the veins on the leaves, but they vary too.
30 May, 2011
Could be Beattie but another photo would make life easier!
30 May, 2011
Possibly a cornus ?
29 May, 2011