You won't get 40 as an average even with 85% motorway driving; but high 30s should be possible.
The DPF regens roughly once per tank of fuel, and that saps the fuel consumption even with motorway driving.
The 2.4 is a heavy car with a relatively powerful engine; high 30s is not too bad considering.
By the way, just to prepare you (or depress you), I worked out that if you do 20,000 miles per year, at an average of around 37mpg, and with fuel costing on average £1.15 per litre, you'll spend around £2800 per year on diesel! That's £230 per month you need to budget for. I think sometimes it's better not to work these things out you know; it's like calculating how many hours of your life you spend on the toilet...!