Just thought I'd post a follow-up for anyone who may be on a similar boat / same sized crew in the future.

At the end of our 15 day charter we had just under half a tank of diesel left and had only just switched over from water tank 1 to tank 2 (both the same size).

...so in the end had no need for additional fuel or water! The rum on the other hand did need topping up quite frequently!

Whilst we always take care with the water we were not being overly lean and in terms of the diesel the boat didn't have a generator so we were using it for at least 4hrs per day either at 1600rpm (for straight battery charging) or around 2800rpm when the breeze failed us!

Another amazing time in the BVI, even with the period of very high swell (11th to 15th Jan...ish) and over 35 knot gusts recorded on several occasions! Anegada will have to wait for 2018!

Thanks again for everyone's advice all the same.