
Councils now have to write to you as soon as they receive a complaint.
Firstly get in touch with the council initially by phone but also confirm in writing.
Explain that the barking is not allowed to continue after first few barks, and that you are working on stopping the issue through management and training (getting him back into the houses as soon as he starts), but it is a work in progress and they cause is him being unfamiliar with his current surroundings and that with re-training and gaining confidence in the new environment should solve the issue.
Any complainant will be asked to keep a diary of frequency and duration of any barking, so I would say keep one of your own too. If the instances are not prolonged or outside social hours (before 7am and after 11pm) then it should not go any further with you doing your best to minimise the issue.
My own breed can be vocal and the only method that does work is to not allow them to practise the behaviour, so as your doing instantly brought back into the house (mine only have the first 20 feet of garden, so it's easy to hoik them in right away). You may need to attach a trailing line so you can enforce the command to come in the instant he barks.
Is the gate solid so they can't see out, if not it may help to make it a solid barrier.