Sorted! Bumped into a decent bike mechanic by chance at a bike meet near me who used to run a bike shop. He discovered it was only running on 3 cylinders (!), checked the plugs, spark, compression test, adjusted carb mixture (carb number 4 screw had been turned out 4 times, hence why she was guzzling fuel), and synced them all for £30 and now she goes like rocket! He did give me a bit of a bollocking for not riding with my lights on in the daytime and keeping it in too low revs before changing up a gear, he said on the open road don't change up until it gets to 6000rpm when accelerating.
I've also discovered that all bmw drivers with nearly new cars insist on racing you - or trying to. It's quite nice to know a £1000, 26 year old bike can piss them off when leaving roundabouts