With very cold weather predicted I just wonder if it would be advisable
to leave the heating on the programmer but turn the thermostat down a
bit.

You have a choice for the heating. Either leave the system on set to 8C or
thereabouts, or turn the system off and drain it down. What you don't want
is the system off, but full of water. Personally, for 2 weeks, I'd leave the
system on, provided it was a sealed system.

As for the water side, rather than heating side, my system is entirely mains
pressure, so I would turn off the stopcock and open all the taps to drain it
down.

Christian.