am going away for six weeks over Xmas - is it better and/or more economical to leave a couple of radiators on low all the time or to programme the system to come on twice a day?
Certainly better to leave the system on as normal.
Each thermostatic rad valve has a little "snowflake" setting ......... called a "frost" setting. Much better than letting the whole house go cold and damp. Quicker warm-up time when you get back too.
I'd agree with that response. If you have a very cold spell during the night when temperatures drop rapidly it will be difficult for the house to warm up again and you don't want to run the risk of pipes freezing up, and bursting.
Far better to set the thermostat at a lower level and be sure that the house maintains a minimum temperature. If it's very wet, that will also help keep any damp at bay and ensure that the fabric of the building, i.e. walls, do not lose too much of their heat.