PIC16F Timer 2 one shot not running
So to sum up.  Always Always Always set TxCON LAST.  The setup order of the other timer registers DOES MATTER but it depends upon the timer mode.
When in one-shot mode, the timer DOES NOT set the interrupt flag.
As a result I had to find a different way of solving the problem - details given in my other posts.
Thanks to Ian Bland and James for your contributions.