Even with the hole cut in the pax footwell, it was still a scrabbling around under the car 2 man (well 1 man and his wife) job.
In case anyone is interested, I can now confirm that it is a square 11mm key, found in Halfrauds under the title of drain plug spanner.
Frustratingly, after doing all of this, I jacked the back up to do the same with the diff, to find that the plug there is a large hex one (I'm guessing 10mm, from all the other postings) and there's only about 1" of clearance between the diff and the de-dion tube. At this point I gave up and started packing for my holiday, I'll do it when I come back.
Any tips about how to get at the diff plug? Is there ANY other way apart from cutting down a 10mm allen key?