Actually the question is somewhat academic now as I've decided not to go with the sub-panel option. Nevertheless, it poses an interesting question for any similar scenario in the future.
So purely for sake of argument.......
What sort of length of run are we talking?.
Rough estimate: Total run from main panel to water-supply bonding point about 100 ft.; sub-panel about 10 ft. away from the water end.
I was planning a 50A sub-feeder on 10mm cable, which under our Regs. also happens to be the minimum size needed for the main bonding on a TN-C-S/PME system.
Given the cost of cable now and the fact that the sub-panel is right on the way, doubling up could have saved 90 ft. of 10mm earth cable, not to mention easing the conduit fill.Edited for typo.
[This message has been edited by pauluk (edited 12-07-2006).]