So about 60 miles on a tenner? Thats about 35 to a gallon, I think you maybe confusing expensive fuel with low mpg. The best way to test is fill your tank to the brim, check and record the odometer reading, drive til the tank is almost empty, fill again note down how many litres it takes to fill,convert the litres to gallons read the milage on the odometer again and do the maths. I've been doing this on my standard saloon since February and am getting almost exactly 30mpg, over a lot of tanks miles and differing journey types. I too would like to increase my mpg, but the consensus on here seems to be 30mpg is reasonable, others seem to achieve 40mpg, just wish I could!!
For short runs - you are doing well! Is the engine in generally good condition? Winding 'the nut' right to the top is not necessarily going to give best mpg. Check the air filter element is clean. Set the mixture correctly (nice even idle). Check the vacuum advance is working as it should (suck the pipe - does the dizzy baseplate move?) - and then set the ignition timing to 'not quite ' pinking when accelerating in top gear. Consider changing the final drive to a 3.9 or a 3.7 (may struggle on a standard 1098) - although any improvement is unlikely to justify the expense.