Ok, so today, I've done a bit of experimentation. I started off taking 50mls of the very sweet/sugary melomel must, and then added 50mls of water. The gravity of that measured at about 1090 or so.
The next was 60mls of must to 40mls of water, then 70mls of must to 30mls of water etc etc.
It looked like the best mix would be about 75/25 as I still wanted reasonably high gravity, to end up with a highish alcohol level, but still quite fruity tasting.
In the end I just put the must in a fermenting bucket, and added water at 1/2 litre at at time checking the gravity in between.
I ended up making it up to about 6 litres total volume, which gave me a gravity of about 1.130, which is still a little on the high side but not unmanageably high.
I've reserved 2 x 500ml plastic beer bottles of the original must, which is sealed and in the fridge.
I've also mentioned to a friend about really wanting to use K1V-1116 yeast, but can't as I haven't got any at the moment.
I explained roughly what's available, and it seems that I'll have to check, because if I've got a pack of red start montrachet, I'll use that, if not then it'll have to be EC-1118 which I definitely have got. I don't like using champagne yeasts on musts that have a fruit element as they do seem to blow a lot of the flavour/aroma straight out the airlock during primary fermentation, but as I've got a litre of the must reserved, I can use that for topping up and/or back sweetening.
I know that this will be a batch that will need a long ageing period, but I'm prepared for that. I'm hoping that something like 3 years plus, should do the job....