What fuel returns are you getting and how do feel about the pulling power at 44 ton?
Im sure asda will put a end to your trauma with their wizzkids at asda house ,i worked for asda 13 years ago and fuel is all they are concerned about ,not wages they pay 
We run 440’s, but the few 400’s i’ve driven haven’t really felt any different to the 440 powerwise so presumably its a matter of mapping.
@ 44t nasty hills on junctioned A road route, as low as 5.5, same weight motorway running from 9 to over 10 driven sensibly.
At George (only an agency scumbag at the time) i’ve seen well into the teens for a days work, kept to 50mph max and driven manually using 1100rpm max, those 3/4 ton loads can see staggering economy.