That's an ingenious adaptation to produce a mobile toilet. I'm an engineer, so I'm used to coming up with solutions and I'd be really proud of that one. In fact, I'd probably file for a patent if it were me.
Aye mate ingenious indeed!
Did use the facilities meself a few times (you had to) and was quite a daunting experience rattling down the M1 on that lower level by the doors where I dunno if it was just me but the fear of those doors opening was telling me bladder that it wasn't quite full enough!