It's actually plumb & true or it wouldn't be possible to use it. The "lean" is an optical illusion caused by my camera & the angle the shots were taken. I did consider editing the image but decided not to. You're correct about the stones which are on the upper floor. Only one of the 2 pairs of stones is in regular use.
Thanks for the link.
This type of windmills is prone to falling over... with the stones and the sails on the same side of the structure, they tend to lose their top when they deteriorate too much. Lots of windmills in belgium met this fate