Whilst stationary in a traffic jam, I turned my mobile phone on.  I didn’t text, make or receive a call.  The phone was in my hand for the split second and I was not moving at the time.  Can I still be prosecuted?

Yes.  If the device is hand-held, even turning the mobile phone on is “using” the device which contravenes the current legislation.  The fact that you were stationary in a traffic jam makes no difference, as you were still “driving” the car.  In order to avoid prosecution, you should only use a hand-held phone when the car is parked and the engine is turned off.