Heres what we did when we tanked a room that suffered from damp
1. Chipped all the old plaster off the wall (Very messy job)
2. Had the brickwork skimmed with cement for a more even surface for the tanking to coat.
3. After the cement dried, tanked it (two coats in opposite directions - up and down, left / right strokes) This ensures you wont miss anything.
4. We then decided to dot and dab plasterboard over the tanking (Leave an inch gap from the bottom of the plasterboard to your floor) This will allow air to circulate and stop any damp that may come from the floor upwards get into your new plasterwork.
5. Had the plasterboard coated with plaster finish.
This was almost 3 years ago and touch wood, no damp has come through.
You could contact a few companies who specialise in tanking to double check, i am only posting what worked for me.
I was led to believe (by professionals in my town who give me a quote) that plasterboard is seperated from the wall with the dot and dab creating an extra cavity as such allowing air to dry any moisture and providing you with an extra meths of insulation??
Some info can be found
http://www.tankingslurry.com