The new charger is 20a. I'm guessing 6mm but I don't know how to measure it. It's thick wire, thicker than might be used in a house, for example, and was connected to the charger that broke, which was also 20a. Would I be right in thinking that if this wire is a little on the thin size the likely result would be a voltage drop, rather than something more serious?
According to Sterling manual 6mm2 is good for up to 2 metre cable run even with their 30 amp charger. Personally I would have used at least 10mm2 with fuse.