I'm going to experiment with an alternative non chemical approach - I microwave old potting compost to re-use, and if I find any VW larvae, am going to scrape out the compost and zap it. Will report back.
Ziminuk, they could be chafer grubs / cockchafer larvae which are very like stag beetle larvae but don't grow so large. You might need to do an internet search to try and compare them though in photos it's very hard to tell the difference.
Ziminuk. This link is to the People's Trust for Endangered Species and talk about stag beetle distribution. There are also maps of recent distribution because the PTES have an ongoing survey where you report sightings.
I'm waiting for the larvae I accidentally uncovered last year to pupate then emerge, so I can report them. They were at least 2 ins long and very fat so I'm hoping for this year.
Excitement over!! I have just trawled the internet and I think I have cock-chafer larvae. I got the impression that they are unlikely to cause too much harm though? However, I have been on the Peoples Trust for Endangered Species Web Site and I understand we do have stag beatles here in Cheshire! How exciting is that?!! I will look out for them.
Thanks so much for all your help Flobear!