Hi All, Thanks for all the replies, which were really helpful and gave me a lot to think about, with points in favour of both the D300 and D700. Mulling over the replies gave me a few more things to think about given that my main photographic interest is people and my main hobby is merchant shipping.
At present my 24-85 f3.5/4.5 (36-127 on DX) + my 58 mm f1.4 Nokton gives me a nice portrait lens set and my 70-300 coverage up to 450 mm which is as much as I need for ships. I was going to invest in a 24-70mm f2.8 (36-105 on DX) for portraits and the 70-200 + 1.4 converter (105-420) for the ships over the next 12 months. If funds permit I could buy a D700, but this would depend on two lenses becoming available. 1 - a 28 or 35-105 f2.8 or f4 and 2 - an updated 80-400 with AF-S and VRII. My reasons for this are that 70mm is too short for portraits on FX and the current 80-400 VR system is not good enough for the conditions under which I use the 70-300.
As I said in my original post, if Fuji do produce a S6 based on a D300 body with a next generation Fuji sensor then my problem is solved.