I've been delving about but am still a little puzzled by the blue colour.
It could be vitalba, (travellers joy or old man's beard) which is a UK native, it meets all the criteria except the colour which should be creamy white. It could be terniflora - some of ours have more sepals 5 or 6 apparently, but that is also white. I also wondered about Summer Snow (x fargesiodes) but that is also white!!
I have written to the National Trust who manage Snowshill Manor to ask formally. Summer Snow is a possibility a lot of their plants were something Snow or Snow something, because of the Manors name. The last resident owner was a bit of an eccentric - he lived in the stables without electricity or running water but had restored the Manor superbly - to house his absolutely enormous collections. He designed the gardens too and apparently spent most of his time there. The National Trust took over the estate about 1950 after he died.
