Thanks David - really appreciate the comment. I've been warming to the lens, to start with I was a wee bit 'eh?' but I'm putting that down to unrealistic expectations.
After last night's conversation with my friend, and having weighed up the kit that I can sell on, I've decided to take up his offer of performing open-wallet surgery. The Df is staying, as is the Tamron 24-70. I'm moving the AF-D primes (50 & 85), the D700 and Pen F on. I've even made a profit on the Pen F, selling it to MPB. Selling the D700 is a bit of a wrench, but with the Df I've found a true replacement for it. I've got the 105-2.5 AI-Nikkor to use instead of the 85, and it works beautifully on the Df.