You can get a cheap alarm form Hawk that is easy to fit. (£20-£30)Good adjustability on the sensitivity, flashing LED, and is loud enough to be a good deterrent with all the hazard lights flashing.That and chain it to the ground.If you're friendly with the neighbours, maybe you could leave the bike in their garage/shed or somewhere behind locked gates out of sight for a while incase the same bastards come back to have another go.