On Saturday I saw a white-flowered shrub on the far bank of a canal.

http://www.stewart.hinsley.me.uk/Images/Shrub05a.jpg
http://www.stewart.hinsley.me.uk/Images/Shrub05b.jpg
http://www.stewart.hinsley.me.uk/Images/Shrub06a.jpg

I interpret it as having opposite foliage, which eliminates all the
various rosaceous shrubs. The venation pattern of the leaves is wrong
for Philadelphus, or at least the common forms (and more obviously the
flowers have the wrong number of petals). I don't think it's Deutzia
either.

At this point I run out of ideas. Any offers?
--
Stewart Robert Hinsley