It's a beauty Paul! I think you will struggle to find which one it is unless your neighbour has strashed the label somewhere, you could try asking if they keep labels. Maybe there's a local seller who knows which ones have been sold in recent years.
You can search on a Clematis site here but in this case all you can really search for is 'blue' for tepals and 'cream' for anthers which brings up a few, more if you only search for blue and several look so much like it.
http://www.clematis.hull.ac.uk...
