Ben wrote:

Hi Martin,

Thanks for the info, I'll give it a go! I think you were right with the over
watering, I re-potted the whole lot about 2 weeks before the rot started,
and probably watered them in too much!

Ahah! That's a serious mistake. Cacti and succulents when repotted you
don't water for at least a week to give the roots chance to heal again.
Same with cuttings you need the wounds to heal and seal before they come
into contact with damp earth. The really rot prone ones you suspend
above damp gravel on net curtain and make them work hard to root down.

Decent sized cacti will survive amazing amounts of time with no roots at
all (I have had some off their roots for 12 months then flower again).
But they really don't like wet feet.

Regards,
Martin Brown