I tried them last year all summer (3 applications - £60) with no obvious reduction in my slug and snail population and tried again just last month with similar zero results. So from a sample of one - my answer is no. I'm giving up and getting a duck.

Depends what kind of nematodes and for what purpose - there are nematode solutions for ants, slugs, vine weevil and chafer grubs. Anecdotal evidence suggests that the chafer grub one works, and possibly the vine weevil one, if they're applied according to the instructions - no idea about the one for ants, sorry.
PS - I've just seen your other question, so presumably you meant the one for slugs...