Drill chuck removal

[the2ems]

The keyless chuck on my Wickes cordless drill has jammed in the fully open position after using it with a paddle mixer attachment. It's out of warranty so no point trying to return it. I bought a replacement chuck from the same store, which it says fits their drills. The problem I have is removing the old chuck. The screw comes out fine, but then I cannot undo the chuck. I have watched You Tube videos showing how to do it, but they involve tightening a large allen key in the chuck and hitting it with a hammer. But I can't do this because the chuck is stuck open. I have tried gripping it around the outside with a mole wrench, but still couldn't budge it. Any suggestions would be very welcome.

Thanks