Thanks for the reply. I am comfortable with using chemicals in a controlled and appropriate manner, as are my neighbours. I would happily try an organic alternative, but even the Corn Gluten Meal seems impossible to obtain from British suppliers.

Has anyone used a pre emergent in the UK and could comment on its effectiveness / suggest a place to buy them?

I'd also be happy enough to try other methods for controlling the crab grass, but short of digging up my entire lawn, what options do I have?