Hi all, well the GS model mystery is solved, it was a ringer!!
So, to recap this was supposed to be a 2005 Japanese import, the frame number matched the logbook and checked out as a Japanese market bike.
But, the more I looked into this the more it was obviously a later bike (probably a 2008) with the front frame loop from the 2005 bike (with it's associated number) fitted.
Clues were the 2008 bodywork, ABSII/ASC non-servo brakes with switchable traction control (which should have been ABSI and servo brakes). Drain plug on rear drive (2005 should not have the drainplug), later (UK) instrument cluster and the complete absence of any Japanese characters anywhere else on the bike (despite there being Japanese characters on the front frame loop), plus a few other clues, e.g. the colour of the rocker covers was different to the 2005 models (a sort of greenish hue), the front telelever arm was black and looked a bit different to the earlier models, and finally the engine number was very difficult to read but the numbers I could see did not match the 2005 logbook and contained EE instead of ED (which I assume also suggests a later bike).
I want to give a good reference to the guys who sold me the bike here (MotorBikes4all in Malvern), it took me a couple of months to learn enough about GSs to realise the issues here so it was a long time before I got back in touch with them to raise the alarm. Fair play, absolutely no quibble from them, they had mentioned the 'updated' bodywork in the listing and assumed that this was just a facelifted import (as did I when I bought it) but as soon as I pointed out the issues they agreed to take the bike back immediately and offered me to choose any other bike from their stock to the same value (or more if I wanted to make up the difference).
So, I now have a genuine 2008 bike (lovely orange colour 'special edition', 53k miles) with all the bells and whistles available at the time (included ESA which I am really loving). In addition, they had all this sorted over the phone for me within a week (which included preparing the bike, fitting new tyres and putting a fresh 12 months MOT), I was rushing them because I was booked onto a ferry for a tour of France and Northern Spain.
The only slight shame is the 'ringer' was a bit nicer than this genuine one, this one has a very rattily clutch when ticking over in neutral that I will probably end up changing next winter (for now it's just part of the 'rich BMW sound environment') doesn't have quite as nice a gear change (the 'ringer' gearbox was amazingly smooth) and has an annoying vibration at 80mph (where the ringer was again much smoother). However, I'd rather have a slightly more 'agricultural' genuine bike than a super smooth ringer that I can never sell on so I'm happy. As you might have seen from my other post, my first test ride on this replacement GS was a 2700 mile ride through France and Spain in 7 days. Absolutely no problems with the bike at all (apart from the racket from the clutch when waiting at lights) so I'm very happy. I'm a competent mechanic (after 40 years of riding) and have a pretty good workshop so changing the clutch, pushrod, etc. shouldn't be a big deal (apart from the painful cost of parts). Looks like a lot of work but pretty straightforward and no more than doing front wheel drive cars and I've done more of those than I care to remember. From what I've been reading the noisy clutch is pretty typical on higher mileage bikes and not an urgent problem. Likely a combination of felt damper worn off the pushrod and play in the clutch plate centre (hopefully not wear on the input shaft but I'll take a look at that when I get round to doing the clutch). I'll probably start by fitting a new pushrod in the next couple of weeks anyway as this looks easy enough to do and see if that makes much difference.