I think you mean head "gasket" - not casket. This is the gasket that is placed between the engine block (bottom part of the engine) and the cylinder head. Your Disco has two heads, so two head gaskets, a left one and a right one.
Depends on if the gasket is the only problem. Can be the head is cracked or warped. Get an estimate from a trusted shop.
The 2002 Land-Rover Range-Rover has 38.2 in. of rear head room.
The 2010 Land-Rover Range-Rover has 38.3 in. of rear head room.
The 2007 Land-Rover Range-Rover has 38.3 in. of rear head room.
The 2011 Land-Rover Range-Rover has 38.3 in. of rear head room.
The 2012 Land-Rover Range-Rover has 39.3 in. of front head room.
The 2005 Land-Rover Range-Rover has 40.3 in. of front head room.
The 2010 Land-Rover Range-Rover has 39.3 in. of front head room.
The 2008 Land-Rover Range-Rover has 39.3 in. of front head room.
The 2009 Land-Rover Range-Rover has 39.3 in. of front head room.
The 2004 Land-Rover Range-Rover has 40.2 in. of front head room.
The 2004 Land-Rover Range-Rover has 38.3 in. of rear head room.