Page 33, 'The Highway Code 2016'
"85. You MUST NOT carry more than one pillion passenger who MUST sit astride the machine on a proper seat. They should face forward with both feet on the footrests.
You MUST NOT carry a pillion passenger unless your motor cycle is designed to do so.
Provisional licence holders MUST NOT carry a pillion passenger.
Laws RTA 1988 sect 23, MV(DL)R 1999 reg 16(6) & CUR 1986 reg 102"
There you go! One of the 'trick' questions asked by examiners is "What are the three things you must ensure when carrying a pillion passenger?" "Facing forward!" is the part of reply often forgotten.

Purple Helmets take note!

My regards, Bill