If I've added up correctly running with a 25 ohm resistor in line from a 12v supply will give 10V across the fan, and running with a 16.7V resistor in line will give 11v across the fan.

Yes you have added it up correctly. I did think you were wrong as you used the starting current instead of the running current, but I have tested with a 22ohm 1/8w resistor which gives a 2.5v drop = 9.5v for fan. so your right, i will try to get either a pot or resistor of the right value tomorrow :)

Could you explain this for me please, I didnt do much at college on motors although this was mentioned and ive forgot it.
the fans I have definalty increase current as the voltage increases. With DC fans the speed increases with voltage, so i can understand it needing more power.
were you thinking of AC fans where the voltage doesnt affect speed. If you know, how would AC fans bennifit from higher voltage (still single phase), would it give them more torque or something?

I also ordered another heatercore today. Last night I used a car parts searching website and in the morning I got woken up by a shop who read my request, and wanted to sell me a heatercore. they had astra and nova ones, so I got another nova one. the astra might be better but I wanted it the same size, as im building a custom case for them. Also tested the fan ok at 24v using -12 to +12 from my atx psu. Now i need to find out how i can run it at 30v+ as its not THAT loud :D