Sunburn could have happened if they were put in sun suddenly - were they indoors before? If yes, they should be put in shade first and slowly moved into more & more sun.
From what you said, they are in good location (midday sun is usually too hot).
They could have some fungus on them - try to wipe the spots with cotton ball-soft cloth or tissue paper; if anything comes off (without too much force) it may be fungus.
Oldest leaves naturally die off, but there shouldn't be many at the same time. If the yellow leaves are also soft, that is often caused by too much moisture. There is a possibility that the mix is not draining well because of sand - if very fine, it clogs the spaces between other particles, generally compacts and doesn't drain well.
These are few things I can think of...

Thanks for your reply Rina!

They weren't put out in sun immediately, no, the bigger one I bought about a month ago was sitting in the sun in the garden centre so if anything, it's getting less sun now than I suppose it did there!

The spots don't wipe away either. I couldn't get a close enough picture to show clearly but the spots are right under the surface layer of the skin on the leaves. Like something bad is inside them, growing out. And yes! The yellow leaves are quite soft! Too much moisture it could be! Although the soil does dry up quickly here, the air is so dry as well as the sun beating down, I've dug up a little of the soil a few times to make sure it's completely dry before watering again and the sand in the soil seems more like grit or even glass than actual fine sand. Although I'm gonna just leave it there in the same place and not water it for a while to see what happens.

In the conditions of my patio as described and with proper soil (I think I'll have a look for better soil...) how often do you think I should water it...?

Thanks so much! (:

Hi Becci,

With my succulents, if I have to use a potting mix of any kind, cactus or otherwise, I will mix it with about 70% perlite. Mix well then use that for "soil." Then lay pebbles or what have you on top to keep the perlite from floating up. In 90F or higher weather, I would water that once to twice a week. In less than 80F weather....maybe once every 10-12 days.

Ideally, I would recommend looking up and reading about gritty mix on this forum. :) Great stuff. Most, if not all of my succs, I've now been using a big chunk gritty mix with lava rocks, sea gravel and super coarse perlite. :) Those, I water every 2-3 days.

Becci
I am glad Sugi answered - her climate is probably quite similar to yours. Try as she suggested, I am sure you'll see improvement.
Just a tip about watering: maybe you can try using a skewer or wooden chopstick, insert about halfway or bit more into soil. If it comes out with particles sticking to it, or feels cold/wet when you touch your cheek or top of your hand - there is still enough moisture. Otherwise, water.
Rina