Finally I got to the bottom of this issue. There may be a warning here for others too.
The fuse blew again the other day, and repeatedly on replacement, within a few miles riding. The first start was to see what fault codes the ABS was storing, if any. Anyway, there is a metal plate fitted under the front seat right at the back. This guards access to the ABS diagnostic connectors. Mine is held in place by a bolt at the top and a tamperproof (one way bolt) at the bottom (presumably this is normal even if it is a little peculiar). Not only is this a tamperproof bolt but it's fitted with some threadlock as well so they really didn't want anyone to be removing it. I had to drill the bolt out to get the plate off. Removing the plate instantly revealed the problem. The plate was clamping down on the cables to the diagnostic connectors (it looks like this is deliberate) and over time this had caused the insulation to become damaged and a short to occur. It looks like the arrangement is intenational, even if it is a little dumb from a design perspective. The short is definitely the cause of the fuse blowing so hopefully now it is repaired and cables re-arranged the problem won't reocurr.
BTW, reading the manual thoroughly there is all sorts of diagnostic information available from the ABS, but the long and short of it is if you have a problem and it's not a sensor or cabling issue, it's likey to be a problem with the ABS unit. This is a single device with everything in the box. There is no serviceable parts meaning a replacement is the only solution. The cost?.........£1144 !!! Best hope it never goes wrong.