Malcolm wrote:

snip
Any soil you bring in, whether turves or pot plants, the latter
probably the most common method of transport.


Back in 1980, when the problem was relatively new, the advice I was
given was that I had probably picked one up in the wheelarch of my car
while travelling through the Carlisle area (there was a known
'infestation' there at that time). The first one I discovered was in
the flower border adjacent to my garage driveway.

--
ned