I'm guessing that I need a special tool to hold the disc still while you undo the central nut? If so is it an easy tool to get and/or can you do a DIY fix to get round it?
Hi David.
If your hand brake is properly adjusted, then Pulling it hard should provide adequate friction to hold the disk still.
You should first make yourself a punch to knock out the dent in the wheel nut. On an bench grinder, or with a file, shape the punch so it has a width that fits in the grove on the axle. In the other dimention make it wedge shaped, so it will push out the dent when you pound it into the grove. Once the dent is out the bolt can be removed fairly easy.
It is, however, not easy to get the dent straightened out. I chose to buy new nuts from Simons and use an angle grinder to CAREFULLY remove the part of the nut with the dent. If you at least grind it down, without reaching the threads on the axle, then using the punch becomes much easier. I would not try to re-use the old nuts.