Syringa variety

A question to all syringa (lilac) lovers.

I came across an outstanding lilac bush growing in an alley near Langstone. Its flowers were of a rare deep violet colour and about twice the diameter of a common lilac .

I'd like to have this variety in my garden but it is not easy to identify it. By the colour (not fading when dry) and shape it looks like syringa hyacinthiflora and is similar to either Pocahontas (not offered for sale in the UK) or Clark's Giant (very rare).

Would be grateful for any ideas.

Also if anyone around Barnet/Hertfordshire can offer cuttings of a dark coloured or unusual lilac - I'll be happy to travel.

Thanks in advance.