there is a chrome strip at the top of the back of the boot lid. The alloy on my lid has corrosion bubbles where it is folded and meets the plinth. To get a proper paint repair I am sure the plinth has to come off. How is it done?

 

You're correct that a permanent repair needs the plinth taking off. As I understand it, the plinth is fixed to the vertical (plastic) part of the boot lid, which is then bolted, (or screwed), on to the horizontal (aluminium) part.

I believe poor assembly line techniques on early cars led to this corrosion site, but its too late now for any redress. You will need to take off the inner trim panel on the vertical section to access the fixing screws. Whether you have to split the horizontal from the vertical sections I don't know, but I think it is probably necessary, as I suspect the problem started at the screws fixing the two panels together.

 

The plinth is a 5 minute job to remove (swapped mine from black to chrome).

You only need to remove the carpet lining from the boot lid.

The plinth has bolts on the back that pass through holes in the boot lid itself (not the trim)

There are access holes cut in the bootlid so you can get a socket (10mm?) on a 1/4" drive in there pretty easily. The fiddlyest part was getting the wiring disconnected for the button.