I was working on a traffic census, where vehicles were stopped by a uniformed police officer in order that we could ask the drivers a few quick questions about their journey. The sole police officer on duty told a driver to pull over to the side of the road, as he'd seen him using his mobile phone while approaching the census point.
As the officer 'issued a ticket' the driver protested that he'd only been using his phone while stationary in the queue. The policeman told "I saw you using the phone while driving. I don't need any witnesses but, if you want to take it to court, I'm sure that these survey staff will back me up".
That policeman (from the Norfolk force, but working in Suffolk) clearly didn't think that he needed another officer to be present.
In fact you don't need ANY police officers to witness the offence. A camera can do the job:
http://news.bbc.co.uk...anchester/7994449.stm
Chris