Hi Supercharged - hmm, nice try, but I don't think this is the cause of my problem. Somehow the gearbox oil is getting onto the driveshaft, then it gets thrown off the brake disc.
This is how it looked after a run earlier this year (I de-gunked all the oil off before the run).
You can see my electronic speedo sensor, but the mounting flange and circlip are not visible. The speedo drive pinion is mounted at an angle and is therefore above and behind the halfshaft. If oil was coming out here it would have to dribble forwards as well as down, crossing the halfshaft flange onto the halfshaft, then down onto the disc, which seems unlikely. The reason why my old gearbox had to be scrapped was because the old speedo drive was corroded into the casing... even though there was no evidence of oil around the speedo drive there was still gearbox oil around the halfshaft. So I don't think it was/is a leaking speedo pinion oil seal.
Another shot showing how the oil gets thrown off the brake disc when on the move. There is a slick of oil painted on the underside of my engine cover louvre immediately above, and a dark line of oil around the back of the flywheel bellhousing. If I clean this oil off, no more issues from the leak site until I drive somewhere.
Just for comparison, this is what the nearside disc looks like - bone dry.
Sorry about the lens flare on the pictures, but they were only taken for information.
And thanks for your suggestions Super, your input is greatly appreciated. I will
get this problem licked (preferably before the next MoT)!