actually im not so sure knocking on their door is the best way - no matter how nice you are, you are going there to moan and it is therefore somewhat confrontational - i think maybe a letter would be better?
you can con trol better what you say in a letter - you may say the wrong thing, make them defensive and start a row
unless you can somehow bump into them for a chat and casually mention it, such as when theyre in the garden etc
if i was new and a neighbour turned up on my doorstep moaning i would take that as a bit intrusive and pushy, and passive agressive
i may forever remember that person as the one that came round complaining and didnt give me chance.
they may not like being put on the spot with something like that and may become defensive.
a very very polite, non moany or accusatory note, with lots of pleasantries - inviting them to discuss further if they like, might be best?
be careful not to say things like 'why should i have to suffer' and 'but come on!' and 'its not fair' etc,
just welcome them, say you look forward to meeing them properly soon, and ask them about the dogs and say you understand they may be new and setting in, but how you were a bit concerned that they would always be like that, and say that you were woken numerous times, and ask them if they would mind not letting them out after 11pm in future - dont mention the tenancy or noise laws - yet - wait and see if they do as you have asked.