I am attempting to damp proof our basement. (For a plan see
http://www.antony.adshead.dsl.pipex.com/temporary.htm)

The basement is built into earth at the S end and opens out to the
garden at the N end. It is unheated and with minimal ventilation right
now. It gets some condensation and during the recent heavy rain it got
maybe 1/2 litre of water ingress over 2 days.

The floor is asphalted.

I want to use a Platon or Newlath-type membrane on the walls at the S
end of the basement and cut a drainage channel to take away any water.

I have some questions:

1. What is the best way of cutting it to form a 50mm-wide channel?
2. What is the best material to line it to provide a smooth run-off?
3. Are there any cold tar-type products that I can proof it with
afterwards?

tia

Antony