mains powered has been a requirement for years, in case tennant decides to pinch the battery from the smoke detectors, thereby rendering them useless. Can't think when this came into force, but basically it was written to target the landlord, which seems unfair if tennant is stupid enough to pinch the battery. Unless mains powered I think the landlord fails in 'duty of care' ie it is a set of circumstances that is easily forseeable and therefore should be protected against, bit like putting fencing around a building site. You would think a building site is very obviously a dangerous place, but unless you make some effort to keep people out then you are open to litigation should someone walk in, and hurt themselves.
