just answering Red Dahlia on Nemaslug - I have used them over the last couple of years. I apply in Autumn (in the hope that I will kill off some overwintering slugs and any planning to play happy families in the dead leaves) and in Spring. They do seem to reduce the slug population, although of course they don't get rid of them entirely, and I wouldn't want that - they do have a place in the natural order. I garden in the city and there probably isnt a hedgehog for miles. I did see frogs some years ago, but sadly they got that virus that turns them red and died, and I havent seen them since. Have never seen a thrush in my garden. So, despite the best efforts of other birds, the slugs and snails don't really have enough predators and nemaslug helps adjust the natural balance.
The only major downside is the cost really, nematodes are not cheap. They are also quite arduous to apply - need to be mixed with water, watered onto damp soil and the soil needs the same amount of water again. That can mean a lot of trips up the garden path with a watering can!
They need at least a fortnight to have much effect, and during that time the soil must be kept moist and the temperature needs to be above 5 degrees........ so although it's quite mild and damp at the moment, it probably is best to save your pennies for spring.