Thanks again chaps for all the good information.
I did think about the dead leg water wastage.
She'll have to make do with it only working when the central is on.
Thanks for your help gents.
By potable rads you mean connect in to the hot water distribution pipe so that your domestic hot water travels through your radiator first?
My wife want me to replace the existing bathroom radiator with a towel radiator but she wants the new towel radiator to heat up when the hot water is turned on without the central heating.
How does this work?