I recently had this problem with my defender when I'd parked it in an exposed spot in the work's car park. Our temperatures during the night had dropped to -21?c. Next morning my def's (TD5) brand new fuel pump was screeching it's head off. I left it for a couple of hours until daylight and came back to it with a brand new heavy-duty tractor battery (my battery had seen better days too) and it fired up after a couple of tries using the new battery as a jumper.
I went straight to my local garage and put 5L of unleaded in the tank and filled it up with derv. As I have a 80L tank, this gave me a mix of approx 6?25% petrol to derv. Never had any more problems with starting until I re-filled the tank about a week later and the temperatures dropped a gain to about -17?c. Once again, I simply added 5L of unleaded and filled the tank to the top. The remaining petrol left from the first time added to the new petrol will be giving me a mix of approx 7% to 8%. To the best of my knowledge, it is ok to go up to about 10% petrol.
If anything, my TD5 is running slightly better on the mix since I first put the petrol in. I wouldn't do it as normal practice, but only to keep me mobile in this weather.
It's an old truckers trick, and, since I'm an old trucker it works for me.
Ron.