Carrying two passengers in a sleeper cab

I am looking for some help / clarification on this subject… My wife and I have our own business and use our own lorry for moving plant and machinery between sites and collecting materials. We have an 18 month old daughter and are looking for a way to fit all 3 of us in the cab so she can come to work with us, especially when we are staying away. The truck is a Mercedes actros L cab 2012, I was thinking of having a lap belt fitted in the centre of the lower bunk so that I can decide the car seat there and we can both sit in the front, or fit the car seat in the front and my wife can sit in the lower bunk. Is this legal? I have hunted the Internet for a straight answer and it seems a grey area?

into the scrappys for a lap belt…bolt it through the back of the cab in the middle of the bottom bunk,attach seat…job done,move on…or…its not a legal requirement to have seatbelts in anything other than a forward facing seat…a bed is not a seat…end of…google uk seatbelt laws…plain English…plod can try and do you anyway,but that’s what it says in the law.

Although in theory seat belts are not required in non forward facing seats - I.e the bunk … The Police can easily go for a dangerous driving charge for a loose passenger in the cab. It makes the vehicle dangerous.

This is something I checked out with our local Police traffic department recently.

There must be a way to securely mount a child seat.