dc42:
If you use a single 0.4 ohm resistor (or 0.39 ohm, which is the nearest standard value), then it will indeed dissipate around 14W when the motor stalls. If you use three 1.2 ohm resistors in parallel, then each one only takes one third of the 10A current, so they dissipate just under 5W each.
At 10A the PD across each of the resistors would be 4V (0.4 * 10)? Therefore the power dissipated would be 4 * 10/3 = 13.3W. I thought I'd already compensated for the 3 resistors by dividing 10 by 3?
Thanks for the quick reply.