I must say my own experience of councils is that they are keen to ensure you do the right thing ( as you have to pay them money for stuff) but not so keen to get invloved in sorting out others' mess (as they haven't paid? - cynical sneeze - and might not.
However, talking to someone in building control or building regs may be more helpful.
Also, it might be a good idea to broach the subject with your neighbours in some way, as to ne honest if a guy from the council does turn up at their door they might be a bit miffed, to say the least. Easier to have the first conversation prov]bably, along the lines of do they know there is a bit of a gap/hole/ lean and you are a bit worried for them/would they like to have a look & see what they think as they might need to contatc their builders... that type of thing. Like you I try to avoid conflict, but in this instance it could be the hand of friendship...