running main bonding cable along same route as water

(c) is the correct way.

You could probably do (b)

I definately wouldn't do (a)

The job only has to be done once...

Owain

What would you consider to be wrong with (a): just the fact that it's touching the pipe (why, cos it's already electrically connected to it?)

David

Yes, and if there's enough current flowing in the bonding cable to make thermal insulation a bad idea, you already have a problem!

Like has been said enough current in that conductor long enough to give thermal problems you have a much more serious problem elsewhere. I'd go against being under the insulation as the wire is then hidden and damage might not be obvious (vermin nibbling round the back etc). I think I'd go for b) outside but on the climaflex (assuming compatibilty between the plastics is OK).

Wiring regulation 528-02-04 is the one usually cited as disallowing this sort of thing:

"Where a wiring system is to be installed in proximity to a non-electrical service it shall be so arranged that any foreseeable operation carried out on either service shall not cause damage to the other."

IOW a plumber may disturb the wiring, or an electrician the plumbing. So only method (c) complies with BS 7671, IMHO.