Think I could just be paranoid. Need to try it on a decent stretch of road. Only tried it on a short stretch at 70mph, so I guess it could be that this particular part of road is on a gradient.
When I picked up the car I drove it home (200 mile trip 70mph) it returned 39mpg.
Now I'm getting 32-34mpg but as I say it's only on a short stretch.
When I'm not paying attention on trying to get good mpg it still returns the same 24-27mpg so that's not changed atoll.