Thanks for the replies, fwiw I wasn't trying to get away with an unsafe installation, I'm actually pretty darn risk averse!
I had been reading past threads on the topic and thought that some knowledgeable folks had suggested that something other than option 5 was acceptable, but I probably misunderstood.
This is what I was envisaging in this (at present completely hypothetical) situation, my thinking being that the charger would only be used very occasionally (once a week/fortnight) and plugged in for maybe 2-3 hours at a time.
I'm interested in this, because it seems to be the opposite of what is written on this page
http://www.tb-training.co.uk/MarineE11.html#MAINS Electricity
ie, 'without a working earth system the RCD cannot work'.