Cheers - the problem is as I thought, one set of spokes is offset the wrong way in relation to the other.
If you look down the centre of the axle, the head of the 'blue' spoke is one hole to the right of the head of the 'red' spoke, but they enter the rim in the opposite formation (blue spoke to the LEFT of the red spoke). This means the blue spoke has to reach further than it should (so appears to be too short) and the red spoke isn't stretching far enough (so appears too long).
Basically you need to move all of the spokes on one side of the wheel round two spoke holes (in this case, blue spoke needs to go from position 1 to position 2... then the spoke that was in 2 needs to go to 3, etc).
Adam