Thought I'd post an update on this. The dealership took the car in and 'updated' the engine management system. The trip computer now reads between 43 and 46 mpg (combined cycle) and it seems to be comparing well with 'actual' mpg.
One thing I've noticed is that the gear shift indicator wants me to change up earlier now (5th at 30, 6th at 40) but the engine labours a bit if there's any load (uphill for instance), so I'm mainly sticking to my previous habits (5th at 40 and 6th at 50) and it still gives me better mpg. I also pushed it up to 70 on a level road and the mpg still crept upwards, so here's hoping!