Originally Answered: Law on music volume/noise?
Well I don't live in London, or anywhere in Europe for that matter, so where I live, the laws may be different. But where I'm from, there isn't a particular time when loud noise needs to stop. If a person has a complaint, then it's usually addressed by the local police right away, and the police will usually just ask the individuals to keep the noise down, or do something to make it not so loud (For example, if they're being loud in the garage with the garage door open, the police may ask them to close the garage door)
If I were you, I'd make a noise complaint, explaining that you have children that need to sleep. Be as polite and curtious as you can. If nothing is done by the police, then I'd politely ask your neighbors if they can keep the noise down a bit. You can even do this privately, by writing them a letter, politely asking them to keep the noise down at night. You don't even need to say who you are if you choose not to.