I live in one of six flats above shops, and use the door and stairs at street level for access. There is another staircase from the first floor landing leading to a yard and a different street at the back of the building which used to be the fire exit - my landlord's builder has now blocked off these back stairs with a wooden door while he renovates the building, work scheduled for 2 years.  Can he just remove my fire exit?

If the alternate fire exit route is required then he should not block it off, if you are concerned contact the local Fire and Rescue Service and voice your concerns.

Check out  http://www.firesafe.org.uk/uk-fire-rescue-services-details/