The message
from "Andrew Barnes" contains these words:

I have a cellar which has damp wall, not wet, just damp. The existing white
paint is coming off in places and there is a white powdery deposit.

I would like to make it into a pleasant room, so need to tank the cellar.

Can anybody recommend any systems?

Have you garden ground outside? If so, start outside. JCB, deep trench
to below level of found and at least three feet away. Field drain in
bottom, gravel and fill. You probably want it further away in order to
give the machine space to operate. Drain water away to a soakaway if you
can do that, if not, take it to a sump and pump it.

Even reducing the amount of wet ouside will make a difference.

I'm assuming you don't want to go to the extent of exposing the outside
of the wall, though that's the best way.

Tackling it from the inside, get a dehumidifier going to soak as much
water out of the wall as you can. Beg, borrow, or hire a setup for
installing an injected dpc and make sure the holes are drilled as
vertically as practicable.

If existing concrete floor is sound lay polythene DPC right to outside
walls. Lay 2" insulation board on top followed by 2" concrete floor but
stop both 6" short of outside walls. Turn up the loose dpc and form a
concrete drain channel 3" wide and 5" deep immediately adjacent to
outsdie walls, draining into a sump -- concrete side of drain channel to
butt against upstand of the DPC.

Keep that dehumidifier running and note how much water you're getting
out of the atmosphere down there.