We are having issues with our neighbors. I play music from my iPad at night to help me sleep, using headphones, and my neighbor has been hijacking it and playing discordant sounds to wake me up in the night is this illegal?
This would be unauthorized access to a computer. The offence is found in Section 1 of the Computer Misuse Act 1990.
Note that this is a criminal offence, approaching the police about it might be the best way of handling the situation (if you want to go that route, and also, I'm not a lawyer).