That looks remarkably like some sort of clematis in flower, possibly an alpina variety, rather than the tree itself producing flowers. Check whether the stem that's flowering isn't part of the tree itself, but scrambling up through the tree branches.

Looks like Chimonanthus preacox to me. And the smell is really fantastic:)

Ah, just came back on to say it might be Chimonanthus praecox, but Kasy beat me to it - I can't tell if the flowers are white or yellow in that pic, and they look quite large, though its hard to tell scale. Chimonanthus gets about 2 to 4 metres tall with a similar spread.

Hi Bamboo & Kasy,

Just googled you are both right thank you very much. The smell is lovely. My back fence came down Saturday and i found this growing on the other side. Now i know what it is i can go out and buy one. Thank you.

Just be aware, Pope Dot, that Chimonanthes can take a long time before it decides to flower, sometimes up to 10 years. The rest of the year, it's not a particularly attractive plant...

You're welcome Pope dot:)
I agree with Bamboo - it's looking good in winter - for the rest of the year it is only a shrub. But it can grow big - one of my customers has got it in her garden. Already about 3 meters tall.
And good luck finding and buying one - they are not widely available in garden centres.