Double checking with a test meter again, you are right, they are reversed. Swapped the wires in an extension lead (fan leads are too short for printer anyway), but now when I power on I only get 21v and fan still doesn't turn. At first I thought it was resistance in the extension, but even if I measure it at the break out board on the gantry, it is still 21v, Also voltage seems to be constant at the pins, I thought it only came on when the hotend was hot. Previously voltaage was a constant 24v no matter waht the hot end temp, now it's 21v no matter what, but I've not touched anything further back than the breakout board.