Well, I bought a brand new high spec air gun (a KS Tools 515.1210 MONSTER-Impact wrench) with 1650Nm of torque in the reverse direction, and that crank pulley nut still wouldn't budge. I think the problem was that the compressor couldn't maintain it's peak output for long enough. It's an electric driven motor from Metabo's basic line, 50 litre tank.
The problem is that there is no electricity at my lockup garage, and I had to run the compressor and fill the tank at home, then wheel the compressor to the garage and use the air stored in the tank to run the air gun. As the pressure decreased, so did the power of the air gun. Had it been run off a permanent supply and I could have just sat there hammering away, I think the result might have been different.
The only option I can see now is to winch the engine into the van and take it to my local garage. I've just about had enough...