Are they a legal requirement on a mid lift axle? My understanding is all the wheels must have spray suppression on a standard 6x4 tractor unit! And a missing mud flap is a pg9 issue if pulled by the V men! Is this correct? 
Most of the ones I see tend to have a small flap on the middle axle, I guess it’ll help in preventing the drive equipment getting covered in crap when the axles down.
I just had to go and have a look lol!! I’ve got one.
But I see many tippers with those big metal fixed ones either side of their dual axles and nothing In the middle which is where a mud guard should be. Interesting one.
I have a Daf with the little lift axle and there is no spray supression on it at all and its just passed its MOT like it
I’m only asking because the lorry I’m driving is on lease and has been returned after 6 weekly checks and still no mudflaps fitted! I was told by another driver that they were probably ripped off by the day men catching them on pavements on delivery of our pallets! 
The other Reno has them fitted but the Boss don’t want to pay for new ones! 