Well I am glad to have found this forum, although it confirms my worst suspicions.

Fact is I now have suckers popping up all over the lawn and towards the house (it's ranged about 15 ft from the original tree which last winter I cut back to a stump.... but that set it into survival mode and its voracious suckering commenced. I have a further smaller one (5ft) - which I appear to have succesfully seen off using a weed killer (on the leaves).

I shall be using the same method (weed killer) on the larger tree bark and leaves - and also spending a good week digging every single last root of this invasive. Will let folks know how long it takes to see off completely/progress on the eradication!