Nick Williamson wrote:

Hello all,

I'm growing some cactus from seed. They popped up in early November
and are currently in a seed tray, in John Innes seed compost with a
layer of Vermiculite on top. The biggest one is probably not much over
half an inch high. I don't know when to repot them, or what to repot
them into - would they be ready for cactus compost yet?

You can get away with leaving repotting them until after they start to
crowd each other.
A nice guide to seed raising at:

http://www.teessidecacti.org/BCSS/CULTIVAT/seedcult.htm

Cacti seedlings will tolerate being kept in a propagator and never quite
dried out for the first year or so. They don't like being saturated but
they grow best at first if kept supplied with some water. At this time of
year they will need as much daylight as you can give them or they will go
leggy and weak.

I usually prick out cacti seedlings after a year, and then pot them up
individually after 2 or 3 years.
Fast growing ones can be done sooner. Cacti seeds are best planted early
spring to get a full growing season in strong sunlight before they have to
face the inexorable grey sunless days of winter.

I usually grow my cacti seedlings in JI #2 cut 1:3 with coarse grit sand
for faster drainage.
Normal seedling compost is a bit rich for them. Potting on into 1:2
JI/grit mix later.

Regards,
Martin Brown