Just to say to all that I have succesfully removed my centreboard afloat and single-handed too. For those who may be interested, I removed the centreboard nut on land then moored up (about 4m from the launch trolly). The removal was very straightforward (but watch out for the thwart where it meets the centreboard case slot). Water did come in once the board was out completely but not at a terrific rate and there was certainly enough time to get the boat back on it’s trolly and out without rushing too much.
Thank you to all for your help and suggestions, I’ll let you know how it goes getting it back in again, which I may well try dry. 😀