That's a beautiful Clematis! If you don't get an ID here, try the Clematis forum at the link below.

Here is a link that might be useful: Garden Web Clematis Forum

Maybe C montana 'Jacqui' or 'Mayleen'? Both are doubles.

Or it might be a chance seedling, of no cultivated variety, an open-pollenated hybrid perhaps.

'Mayleen' is single, but highly fragrant. I once grew it.

If it just appeared, it would likely be a seedling as Richard mentioned.

Oops - mislabelled pix strike again. You're right, of course.

I'm not completely convinced by the seedling idea. I've got a 20 year old C montana and it has never produced a seedling. But if it is a seedling, quite a find.

Could be 'Marjorie' or 'Broughton Star' - both are light pink doubles. And I share floral's skepticism about the seedling. I've seen a lot of very venerable and large montanas over the course of my gardening career but have yet to see a seedling. IME, anything other than our native clematis do not self seed in this climate, which should be somewhat similar to the UK.