Thanks again to all.
I've now ordered a couple of the height corrector rods from eBay.
However they show nothing for a Rear Entrance Post Plug (as so described on the parts list - slightly unfortunate choice of description I feel)
So I'll beaver away on google and see what I can come up with!
Alasdair - thank you for that kind thought - something you can be sure I'll bear in what little mind I've got left.
The hole you have shown in your photo is not the rear entrance post (which is an oval hole, not square), I think the hole you have shown is at the top of the wheel arch and has no plug.
The REAR END plug (I am such a child) is obtained only from a main dealer, I have not seen them on ebay. The sill can/could also be sealed with a mastic barrier.
I do not think it is that which you are after however........