Guh, I feel your pain as we used to have a neighbour with a non-stop barking dog who would go nuts when there was no-one at home.

I'm not sure what legally permissible options you have to make the dog stop now. I think the next best thing you can do is to keep accurate and diligent records. Make a noise diary and note each time the dog starts and stops barking. Contact your council and get them to begin an investigation. Maaaybe contact the RSPCA and tell them that you suspect that there's a neglected dog and you're concerned about its welfare? <shrugs>