Ken there is no Correct spacings---It will depend on the joists above if 400mm centres then at 800mm intervals will be ok. At 600 centres the spacing will then stay at 600mm or increase to 1200mm. If the bathroom is on the ground floor then fire rated D/lights would be the route to go down. The general spacings from walls and all flammable objects is 500mm. Hope this helps. Also this type of work is notifiable to building control and the work must be carried out to 17th edition electricial regulations.
