Thanks very much, Sam, that's a great help.
One other thing I meant to check on: if my partner did buy me out of our current property, would the amount he gives me for my share be subject to Stamp Duty? That's the impression I got. (But Stamp Duty just on the amount he gives me, NOT the whole value of the property?)
I must admit I was hoping to be sole owner of the investment property but it doesn't sound like it's the most efficient way of doing it.