I have done more research and it isn't crabgrass that I have. I was convenced it was crabgrass after a friend had a look at it and said that he had the same at his last property.

It is just moss. Tonnes of it! Perhaps there is an organic substance that I can use to kill it? The stuff I used last tie made it go quite black in patches but didn't stop it returning. Perhaps I am cutting the grass too often?