Mouthwashes can and usually do, contain Xylitol which is very toxic to dogs so never use one. I would contact the vets for advice on what to use to help.

You could try Vet Aquadent available from www.vetuk.co.uk which contains the active ingredient in Corsodyl, chlorahex thingummy. It is specially for preventing plaque build up in dogs. Although, interestingly, it also contains xylitol.

It is about twice the price of Corsodyl if you get the small bottle, but the big bottle is much better value and you only need to use half a capful in their drinking water, so it's not too bad.

Thanks for that Jet. The only reason I was asking is that a greyhound website said to use corsodyl mouthwash to rub on their gums.
I realise that human stuff isn't usually recommended for dogs but it seems odd that they recommended it. I can't remember which website it was now as I'd looked at loads otherwise i'd have put the link up for all to see!