Nematodes

Suspect they are natural to the UK in any case. We have only been isolated from the rest of Europe for a few thousand years. Not enough to have a completely different ecology. What you do is add them to the ground in a concentration so that there are a lot more of them to kill off the pests that you want them to infect. Basically increase the chances of infection.

Also they say not to mix and water in unless the temperature is above 7C as they will not survive. So it appears that they die at low temperatures, like present. I would guess that the eggs are what over winters.

The UK is separated by 22-25 miles from Europe and butterflies cross that gap annually.

May I reply Fatcat?  "They do no lasting damage".  As far as their prey/victims are concerned, your a long time dead.     Cheers,   Tony.