Crankshaft pulley nut - help!

Hi
Series 111 with a 10J engine - any tips on how to remove the crankshaft pully nut (with starter motor extension)?
I can get a meaty adjustable on it with an extension handle but rather than undo, the engine turns under extreme pressure despite being in gear, in low, & handbrake on - taking off the starter motor to get a screwdriver to lock it looks like a lot of work on top of replacing the crankshaft front seal.
Anybody got a simple dodge??????????????????????

on other vehicles-ring spanner and flick starter-provided there is something solid for it to hit (make sure it can't start, remove king lead or disable fuel pump on diesel)
Or
Get someone to press brakes and handbrake on while you undo it

buy a ring spanner or socket-adjustables crap and damage nuts

Cheers - success!
Had a devil's own job to get hold of a 1 5/8" ring spanner (You were right - forget the adjustable) but managed to get one from a mate (none on e bay etc). I put a piece of dowel, taped to the spanner shaft, in the starter handle dog to hold the ring spanner in place and then let it spin a bit to get the torque as it hit a wedged block of wood - I did apply a bit of heat to the nut to help - and off she came. Without the heat and the spin it was going nowhere - just wedging the spanner and flicking the starter didn't work - you need the sudden jolt to free it.
Thanks very much