Derek Turner wrote:

There is a strain called 'Bocking 14' marketed by the HDRA which is much
less invasive and just as beneficial. I've grown it for 5 years without
being driven out of house and home (or garden).

Sorry to follow up on my own post but I've just googled Bocking 14 and
discovered that it's sterile - will propagate through root cuttings but
does not seed (flowers beautifully, bees love it). Explains a lot!