Hi Matt. Where are the photos?

Sorry Anton, thought it had attached them but hadn't done so but should be on now.

Thank you very much palmsandsnow, I've currently got a heat mat so maybe I can chop them and let them dry/callous on that. I have no idea why its suddenly onset, however all I can think is cactus lower body had been covered over soil quite high by the supplier ??

Usually rot will show up quite suddenly, all at once. I don't think a heat mat will be necessary- you need warm, not necessarily hot. I also don't think there is much need for a heat mat in general. Of course, it all depends on your climate. If you can, grow the cacti outside (once they are rooted) in full sun. To do this, you will need to acclimate them to the higher temperature and sun levels by leaving them outside for increasingly larger periods of time. In other words, slowly increasing the light levels. I would start only about 1 hour outside, progressing upward maybe an hour every 1-2 weeks, until you can keep them outside permanently (if your climate doesn't allow this, put the cacti in a south or westward facing window).

Your problem with the rot was most likely overwatering. Although I am sure it didn't seem like you were watering much, with the soil that cacti usually come in (compost, topsoil, and usually a lot of peat), the top couple of inches can be bone dry, while in the middle it is extremely wet. That is what makes this kind of soil so undesirable.

To test when to water, you could use a wooden skewer, and poke it into the soil as far as it will reach, and if it comes up moist, you will know not to water.

Thanks again, no the heat mat just for my other hobbies but could use it make sure it callous over right etc