In message , g00se
writes
Just purchased 7 extremely healthy looking cacti from B and Q

Now that is unusual. But I guess even B&Q find it hard to underwater
cacti.

Unfortunately each cacti doesnt look like it has much room for growth - they
are currently in 3 or 4 inch plastic pots.

How much room should cacti have in order to feel comfortable? Should some
compost be visible?

Depending on what they are 1cm round the edge would probably be OK for
4" pots. They are most easily repotted early in the year unless
extremely pot bound. Overpotting tends to cause rot via wet roots. A
hint that it needs repotting is when roots start coming out the bottom
of the pot.

Also, shall I bother feeding the cacti plant food? Or just water them once a
week or so with normal water?

Don't waste money on specialist cacti plant food, anything with high
potash low nitrate like tomato feed is ideal. And don't over feed.

Perhaps less obviously let the compost dry out between waterings and
give generous amounts of water but never leave them standing in water.
This is a lot easier with very open grit based compost on staging.

In mid summer and active growth they can need almost as much water as
normal houseplants (but will survive with much much less). Succulents
can be a bit more touchy about watering especially in the recent
heatwave.

Regards,
--
Martin Brown