Thanks Nick for your input.
The electrical bonding solution seems to me to be the way to go. The three books on boat electrics all seem to advocate this approach as do a couple of magazine articles. I must admit that I hadn’t come across the inclusion of sea cocks, but it does seem to make sense – but I wonder that if the sacrifical anode is at the after end close to the propeller, will that self same anode also protect the head’s sea cocks 20 or more feet away?
I look forward to getting further views. Once I have made up my mind on what solution to adopt, I’ll let you know. I must say though, that my knowledge of the physics involved just about copes with the elecrical bonding solution, but eyes start to glaze over when stray currents, lightships and galvanic isolators are introduced.