You'll need to get a closer shot Peter, with a plain background - the leaves "disappear" against that hedge and we (or at least, I ) can't see the flower clearly enough.

I agree Beattie

Not very often these flower, they must be starved! Its Ipomea batatas 'Blackie'

Copy and paste link below into browser:

http://farm4.static.flickr.com/3011/2571031188_32eac1ce9e.jpg

Thanks My photography leaves a lot to be desired. But I think Ipomea could be right. Sorry to be so thick but what does starved mean, Thirsty or needing a feed?

Yes, hungry and needing a feed.

Talking of hungry, as these are the Sweet Potato, if they have formed tubers on the roots by the end of the year you can eat them!

Fantastic! I'm amazed that you could ID them Fractal & impressed by the combination of the flowers and leaves. There's more on growing them here -
http://mrbrownthumb.blogspot.com/2007/08/ipomoea-batatas-blackie.html

Good spotting at the car boot Peter! One of my fave places to pick up plants.