I’m putting in a lounge fire, in the form of a contained log burner.

These are the rules I’ve found regarding a hearth.

From http://www.stovefittersmanual.co.uk/articles/hearth-regulations/:

• Stricter rules if a stove into a fireplace recess Vs a freestanding stove.
• Freestanding stove rules (NOT in a recess):
  • assumes stove  is independently certified not to heat the hearth underneath it to more than 100 degrees centigrade;
  • 12mm thick non-combustible superimposed hearth onto the floor
  • Minimum size 840mm x 840mm
  • Minimum 150mm each side and behind the stove
  • Minimum 225mm in front of the door. Best practice is 300mm.

If it’s a stove going into a fireplace recess, also look at http://www.stovesonline.co.uk/stove-hearth-size.html.