Nemaslug / nematode questions

So I just applied nemaslug to my vegetable patch and a few herb pots. Then London was hit by this absolute torrential downpour! Apparently the normal amount of June and July's rain combined in the space of an hour!

I'm worried that the nematodes will have been washed away/ washed too deep? I know you're supposed to water them in to an already watered patch.... but this rain was TORRENTIAL. Has probably damaged some of our crops on its own....never mind what the slugs are doing!

Do you think this is possible, or will the nematodes stick around the right area/depth? We have quite heavy/ clay soil, which I guess might help?

Also, unrelated question, but how long after applying nematodes should I wait to sow some new seeds (probably some lettuce) in the areas where the slugs were previously a nuisance? I know it takes a few days for nemaslug to work, but if I sow the seeds today/tomorrow, there shouldn't be any seedlings/ shoots for the slugs to much on whilst they're still alive right? Or should I just wait until I see a lack of slugs in my garden?

Thanks