Thanks for the further answers. I've not made any more progress with this but am posting to acknowledge your help.
I've sent photos (the same as attached to this thread) and that video to a friend with a better workshop and more skills than me and talked it over with him. His suspicion is that the shroud (better name for the plate that is attached to the outer CV) has been passivated cheaply and is rusting, making it slightly porous and allowing the grease to leak out. It's clear that the shafts are going to have to be removed but he is thinking along the lines of cleaning the inside of the shroud and applying a thin coating of something to re-seal them on the inside - he's suggested an epoxy as it will be strong and not degrade in the grease. Given that the CV's aren't exhibiting any signs of wear (no ticking or clicking on cornering) then I propose to do this as soon as I can before they wear out. (Having said that, it occurs to me that the amount of grease that has come out is small compared to the amount of grease that would have been in the joints when new so I suspect there is a fair amount in there yet).
I am therefore arranging a date with the aforementioned friend to go over there, stick it over his pit, take it apart and have a squiz at what's what. I've bought four new boots and circular clips and have a big tub of moly grease. I'll keep you posted.
Cayman'68 - thanks for confirming the colour of the grease! It looks for all the world like cow poo ....
KKiana - nice to know your solution worked. How did you know the leak on yours was around the edge of the boots?
Update. I went to visit the friend with a good workshop and we did the job.
Three points to note about getting the driveshafts out:
I don't see how you could get the driveshafts out without removing some other bits. I took the lower plate off which made getting the spline bolts out much easier and also facilitated getting the driveshafts out. Unfortunately I snapped both of the frontmost bolts in the process so they had to be drilled out and re-tapped (M10). This took time.
The stub axles came out of the hubs quite easily; a bit of a thump with a large hammer and a drift and they were free.
The outer nuts are done up hellishly tight. We had a 32mm socket on a 3/4in drive handle, with a 4ft length of scaffold bar and me standing on the end and it gently came undone. Someone could do the maths but I'd guess that was a good bit more than 340FtLb.
A very good look at the CV's was most instructive. They weren't as in the video I posted a link to; there was no plastic on them and neither was there an ABS toothed ring. They were a horrible mess and we cleaned them with a wire brush, then brake cleaner, then put them in a sand blast cabinet. They came out pitted and the shrouds were showing two or three small axial cracks from the pressing which attached them to the CV. However they were still firmly attached and couldn't be persuaded off the body of the CV's, which was good. They didn't seem porous (which was my theory) and it appeared that the grease was leaking out of join where the shroud was swaged over the CV body. We therefore warmed them up and put some epoxy glue (slow-set Araldite) around the swage to re-seal it. As the metal was warm the glue was drawn into the gaps very nicely and the end result looked most promising. We then painted them to dissade further rust and for practical reasons put some tape around the ring of glue before re-assembling.
I didn't take loads of photos as I went along but here are a couple, showing the axles with the glue in them while it set. You can see the very clean grey pitted surface and the wet glue.
The boots were fine and I didn't take them apart to put more grease in; I didn't fancy disturbing them and I don't think I had lost that much grease.
I cleaned the suspension members where the departed grease had built up so I can see if any more comes out and will watch and see what happens over the next thousand miles or so. Given that I had the same problem on both sides of the car then I am guessing that there are a lot of cars out there with this. The car has 100,000 miles on the clock and is a late 2007 model, so 13 years old. I'd be interested to hear from anyone else who has the same on the back of their 987.