Thames tap water will make life much easier, thank you Steve.
I share your concern about using chemicals in the garden because of possible harm to wildlife. I am careful to keep the slug clear to a minimum and I collect any 'bodies' early in the morning. However, I have a few plants, miniture hostas and some codonopsis that wouldn't make it through the soil in early spring without a little slug clear. It kills the tiny black slugs under the soil. My back garden is heaven for molluscs, damp and shady but, thankfully, there are plenty of fat frogs and town birds.