Aberdeen cancelled, went to a job in Dartford this morning instead. Still did the test:
221 miles non-stop motorway, cruise on at 80mph most of the way. Both DIS's reset at fill up.
DIS1: 32mpg av, 73mph av, 3.03 hrs
DIS2: 32mpg av, 73mph av, 3.03 hrs
Used 29.7 litres. (221/29.7) x 4.54 = 33.7mpg average
So the DIS measured under the actual mpg, which is nice!
I think this is ideal conditions though with steady speed etc, so i'll measure another tank with varied driving conditions over the next day or two.