Choufleur, we've got exactly the same problem with two neighbours. We've spoken pleasantly to both - on several occasions.
One side (two doors down) used to let their dog out as early as 6am and before and leave it outside while they went to the gym, it yapped constantly for an hour or more.
Then they'd leave it in the garden if the weather was nice on the weekend and it just barked, barked, barked constantly and ruined everyone's enjoyment of the garden - all the neighbours were complaining to each other about it except the owners of the dog!
We spoke to them, it stopped. When it started again, we went round again - we just said we weren't going to put up with being woken up every morning and not being able to enjoy our garden at weekends.
It's better now, they don't leave the dog outside and they take it for walks, which they weren't doing.
Then people moved in two doors the other side with a very large dog that barks so loudly and constantly you can hear it from inside their house - and we're not even attached to them!
Our immediate neighbour is elderly and was very distressed so we went to speak to them - again, politely. It's a lot better now.
Some people just don't realise how annoying it is - I've no problem with dogs barking, it's just when it's so constant it drives you mad. We have a dog who doesn't bark, luckily not even in reaction to the neighbours' barking dogs!
We did register a complaint with env health about the large dog when speaking to them didn't help the first time, but since my OH went to talk to them a second time it isn't a real problem any longer.
I know it sounds moany and we've been unlucky for it to happen twice, but why should you bite your tongue and put up with it? It's a bloody nuisance and really annoying.
Just speak to them, worst they can say is eff off, and if they do, go to env health.