Actually this has improved. Probably due to the warmer weather (hah!) but the dash is now showing over 53mpg most of the time and fill-to-fill I'm getting 52mpg.
That is a much closer match than I get. My fill-to-fill actual was only 2 mpg less than dash on one tankful, but usually about 4 mpg.
I try and be consistent when filling up. I stop filling at first pump shut off, then one more squeeze to second pump shut off. But I fill up at a big variety of places so there may be some variability in how full each tankful really is.
My car has averaged 47.1 mpg over its first 3500 miles. This includes 800 miles with a roof box, running in, and my usual mixture of long and short drives. I do not drive for economy. Best actual fill to fill was 51.5 mpg.
47.1 mpg is 9% better than the overall average I had from my Mk2's 43.3 mpg, except I didn't have the Mk 2 from new, so no running in period.
I am hopeful it will settle down a bit better than 47.1mpg with the running in out of the way.