I will be changing the diff oil in my '90 Majestic soon. I have used the "recommended" process of refilling and it is a PIA...
Not to digress, however, my '95 XJS had plugs in the rear bulkhead under the fuel tank that were easily removed and greatly simplified the diff. refill process.
Are the plugs in the XJ40 as easily removed as those in the XJS?

 

I can't remember whether 40 even had same plugs as XJS but it should be relatively easy to refill the diff, just take the rear right wheel off, car on the stand of course, 1/2 inch extension in the fill plug and off it comes hopefully easily(I've struggled because I've got monobrace which you won't), when that comes off, you undo the drain plug, oil comes out, put the drain back on and to fill just use one of these huuuge syringes with flexible hose at the end for sucking fluids, didn't cost more than a tenner and diff was filled in a few minutes. No need to get in there from the boot.

 

Thank you for the advice. I'm just going to have to get off my *** and get it done.