Hi Oliver. I recently bought a Renault Clio, french diesels seem to top the charts when it comes to economy. Only managed 60 to 63 MPG in mine thus far, but hopefully I too will be able to get it into the 70's/80's once ive perfected the technique!
She's got less than 6k on the clock so im expecting it to improve with the mileage. Ive tried a few techniques but its difficult here as I live in Wales, probably similar to where you live, there are no highways and lots of hills! Plus during the week I do mostly short trips to work and picking up ky gf from work too.
The most I had was 94.6 MPG but that was only over a 16 mile trip so cant take too much notice of that!