This was a few years ago so there may be gaps in my knowledge. I took the photos then and promised to write up an instructable 'soon'. Better late that never I suppose.
Step 1: Get Cutting
Using a sharp sturdy craft knife cut out a small section of the rounded section to reveal...
Step 2: The Secret Locking Mechanism
Using something pointy but not sharp (I used a souvenir letter opener from Madrid), slide the spring and lug away from the centre of the wheel to release the wheel from the axle.
Now you have one wheel free the other wheel and the axle should be able to be removed as a single piece from the other side.