Doing the "rounds" at home this afternoon I found that our outside tap is well and truly frozen up and that the (15mm copper) pipe has split. Good job I found it while it's still frozen.
In previous years I have isolated the outside taps and simply left them turned on and never had a problem. This week I forgot.
The repair is easy (but I'll wait for a warmer day).
Q. Would I be doing myself any favours by changing the offending pipe segment to Hep2O, or MDPE?
D