I couldn't undo it with the belt on.
I tried just hitting it with a hammer while the belt was on but that did nothing.
Then I bolted a piece of steel to the pulley and used a viscous fan spanner. Both of these had extensions on the end for more leverage. I then had a helper and the two of us pushed in opposite directions. The viscous fan spanner cut through the nut without undoing it.
I had two fan spanners to double up but I couldn't fit them in with the steel bolted to the front of the pulley.
So then I took off the alternator, AC pump, PS pump and the pump bracket. With these off I could fit a longer bolt in the back of the pulley, slide a ratchet extension in from the top that was braced against the alternator bracket and spin the pulley so the bolt rested against that.
I could then fit my doubled up spanner on the nut to spread the load and really whack it with the hammer.
Took several thumps but it eventually went.
The pulley on the PS pump bracket had to come off to allow the bracket to come off.
The other pulley had to come off when I was removing the water pump. It was covering one of the bolts and I couldn't get a socket on it.