Thanks for your replies.
Andrew Gabriel: the copper pipes are mains-fed cold water feeding a
tank, then the outlets from the tank. I won't be worrying about the
max exposure temperatures then. I accept your (an Tabby's) correction
about MCB types. I wouldn't be using massive lengths of the stuff. I'm
not keen on home-brew methods, hence wanting a reputable product for
DIY. Not keen on a lack of water either when pipes freeze, never mind
bursts.
cynic: the website I linked to says "Designed specifically for frost
protection and low temperature maintenance". http://preview.tinyurl.com/4gmwfsj
has FS-A-2X Self-regulating frost protection cable 10W/m at 5°C.
What's the difference?
Tim Lamb: by "unaffordable" what do you mean? I'm prepared to throw
£50+ at my problem this year so I have cold water to feed the hot
water system when (not if) the weather turns nasty like it was in
December. I've briefly looked at soil warming cables and they cost
similar per metre.
cynic (2nd post): your explanation is as I understand how they work.