Jules wrote:

Is there any way you can get the height for the replacement door (cutting
the lintel as needed) whilst the current door's still in place? I
think that's the 'scary' bit personally - I just didn't know how long
that bit would take or what I'd find when I started hacking into the
plasterboard on teh one side, or cladding on the outside.


Taking half an inch off the bottom of the lintel wouldn't be easy. Not
enough access for a router or circ saw. Maybe a series of plunges with a
kitchen hinge cutter and chisel the rest.
I'm exhausted just thinking about it :-)