It is a probable cause and in my view it would be worth the investment for a bit of surge protection. Its better than fitting a new boiler only to find you have the same problem.They do however visit the council tidy tip a lot and pick up old thrown out TV's, DVD players, computers, laptops etc bring them home and plugging them in. Do you think this could be the cause.
It crossed my mind earlier that some of this could be a scam to avoid paying rent. The idea of your tenants playing about with the boiler for personal gain is getting a little stronger now. As you know your tenants, do you think they are capable of it?