I have a property where the emergency lighting in a staircase does not work in emergency. I tried to replace them with like for like and when fitted work fine but when I put it in emergency the lighting will only last a few minutes before switching off. Any ideas before I call out an electrician.

From what you have said, you tested it just after you replaced it without giving the battery time to fully charge. I would leave it to charge for 24 hours (to be sure) then turn the mains off again, this time the light should stay on for 3 hours.