If it's the same as a Td5 Disco (which I believe it is - somebody will tell me if I'm wrong) then it's 3 nuts to remove the downpipe and then another 3 off the manifold. You'll have to remove the heatshield and actuator as well.
Not a big job, but the bolts can be tricky to get to.
As far as I know there is no need to do anything with the ECU.