I was hoping to install my own core heater (Defa 256). How hard is it to get to the freeze plug behind the engine on this particular model?

 

Might be easier fitting an Auxiliary heater unless you have already got one and don't know about it. LTD editions had them. As to core plug fit your toiling for room...

 

When I looked into engine pre-heating some years ago for the diesel Jag., the core/freeze plug method along with the dipstick method and the glued-on sump heater wasn't considered very efficient.
Are you quite sure you want to go this way?
The bottom hose method with the major-size electric heater plus an integral pump was considered the most effective. You'll need to put between one and two kilowatt-hours into an engine coolant system -- something like a domestic immersion heater for a bath. These tiny element heaters of a couple of hundred watts or less are not satisfactory, You need to get the coolant above 60*C to pass the engine friction problem, or to have the heater working instantaneously on start-up in winter.
The drawback with bottom hose method is the cost, it's going to be at least £250 for the part, although it's not a hard job to do. Just make sure rain cannot get onto the mains-voltage connector.

Leedsman.
Oh, and as Goggs says, it's a fiddly job messing with core plugs, and I'd bet money you get leaks after...

 

tfb had a second hand 'proper' Webasto axillary heater for sale. No idea if its sold or not. Ask !

 

I'm thinking of fitting the Kenlowe 240v heater I got 20 years ago but never used it on a Rangerover I had. It fits nicely in place of the Auxiliary heater of Voyager that went ***** up on me and nearly lost my engine when coolant leaked out auxiliary heater.
Down side is you need a drive where you can plug it in but I've got one.

 

I may be able to exchange the part for a universal defa 411420.
The advice I had gotten was originally the core heater, but if it is too much work to install I may reconsider.