Swear at the bl----y thing or -
if you get one bolt out, you can use the firewall it's self as a dirty great big lever and twist the remaining bolt with lots of WDCRINORP or what ever fluid.
Watch out you don't rivet the bolt in the hole by using a drift and hammer, as that can mushroom the end inside the hole. a little heat is good but keep it to a black heat and getting it to glow white hot is not the thing to do.
Oh yeah and think of all the want-a-be patron saints of the Land Rover and pray to them.
Failing all of the above one of the stuck halves can be drilled out.