Thanks for the suggestions everyone.

There is a stop tap out there and it is now turned off but the stop tap is
about 12 inches above floor level so the pipe below it is vulnerable - and
it's lead.
It is impractical to run in plastic as the toilet is at the bottom of the
garden away from the house and the pipe emerges from concrete in the toilet
and there are flags laid on the route from the house.

All the pipes are insulated and were last winter but no burst then and it
was colder for longer.

There is no electricity out there at present but I do like the idea of a
frost stat ( have a redundant one somewhere ) and a low power electric
heater.
I think that is the way forward for me.

Thanks again.