Ive been told to spray on a product called Chas 48 available in garden centres but wondering how effective this is. A friend has completely replaced his lawn this year because of these bugs and now uses the Chas 48 to keep them at bay.

Having spent a fortune on turf last year, we certainly don't want to risk it.

I identified the bug positively by the 'song' which is very distinctive. Here is a link: http://www.arkive.org/species/ARK/invertebrates_terrestrial_and_freshwater/Gryllotalpa_gryllotalpa

Mole crickets are easy to treat. They can be eliminated with some soil-applied pesticides such as chlorpyriphos, Lindane, and Sodium fluorosilicate. There are also many other pesticides which are pellet-like and easily dispersing on the ground.most of them are relatively odorless.

But do kill them before they multiply too much.Then it would be late.
Good luck