Thanks for the response, some food for thought...particularly spiritmonitor, as I hadn't come across that before. I understand the scepticism of HJ, but for lack of knowing any other good sources of data, it was all I had to refer to.
The average on SM comes out at 40.2mpg, so I guess I'm 'only' 10% off that. That said, I would say my driving style/usage is relatively efficient in that I don't race between lights, generally try to coast into roundabouts and red lights etc, and I probably do 50% of my miles on the motorway (70-75mph), with a decent amount of the rest on A & B roads, rather than in town. I guess the fact my average over 150k miles in the Accord was +4mpg (+10%) over the spirit monitor average, kind of fits with that.
In terms of things to check, since your message I've checked the brakes and the rears are practically cold after a drive, and the fronts not hot enough to be concerned about. Losses due to pulleys/bearings etc should be relatively small, given if they were that worn, I'd expect some serious nosies coming from them - however, it doesn't do any harm to get them checked, so I probably will at the next service.
I get your point on autos of old, but 'modern' autos are (in theory) more efficient/designed for economy and closer to that of a manual. E.g. Mazda claim the skyactiv-drive auto is "A new-generation highly-efficient automatic transmission" - obviously to be taken with a pinch of salt, but I can't see them having spent time/money developing a brand new transmission in-house, for it to be negligibly better than the one they had before. There's also i-ELOOP regenerative braking and auto-start/stop to add into the mix on my car, which in theory should both result in improved economy over older-tech diesels.
Maybe the DPF is regenerating more often than it should, or the regenerative braking isn't working as it should and is just causing drivetrain losses, but I'm not sure trying to diagnose either of those is a particularly DIY job. I might mention them at service time and see what their thoughts are.
Thanks again.