Aye Rufus,
I get about 60mpg out of the 450 Duke at the moment on too low gearing and a 32mm carb, whilst a friend has a CB250RS that achieves 80mpg regular. So, besides emptying out the tank, I would check out for filters in the fuel tank side of the fuel taps and flush the whole system out and give it a blast with an airline, if you have one. Also check the carb filters for blockages and sediment in the float bowl.
Once you have been through that little lot, put a good quality lawnmower filter between the tank feeds and the carb for extra insurance. Use Castrol Valvemaster for every fill up, 1ml to 1l of E5 or E10.
Carb air filter hoses can cause all sorts of problems on Ducati singles, maybe better with a K&N pod filter straight over the carb intake. I know it's a CB250RS, but, you never know.
Finally, sentence taken from a short online review
Fuel consuption was about 5 litres/100 km ridden hard or as little as 3 litres/100km riden gently.
That works out at between 56 and 95 mpg and I suspect on those Dumfries roads your giving it a bit of welly!
So, I wouldn't think much wrong with the carb, just E10!
Link to quoted review:
CB250RS_reviewAs for the lemmings .....!
As posted in rants!
Good health, Bill