In reply to all:
Can I add a couple of top tips from my experiences? Just got back from Costa B having pulled these off quite successfully....
There are the obvious things that have to go in the hold - but these should easily total less than 15kg unless you're using steel biners - but you should aim to either wear, carry, or take as hand luggage almost everything else - no matter what it weighs!
If there are 2 or more of you at the airport, then leave loads of heavy stuff with your friend round the corner, check-in, complete with small light handluggage rucksac - then return to base and stuff everything back in your bag, pockets etc. Your bag is never weighed or measured after check-in.
If for whatever reason your hold luggage is over the limit, make sure you put it on the scales so that it is leaning against the side - that knocks a few kg of the reading. If you're really struggling then it's straightforward to leave the end of the bag slightly hanging off the edge and then take quite a bit of the weight on your foot. A friend of mine checked in a 35kg bag no problem like this.