Interior tanking for penetrating damp

Following all the heavy rain at the start of the year penetrating damp has occurred in a 1st floor bedroom over approx 2 sq.m just above the skirting. External wall is rendered (seems sound enough), 1880s mid-terrace solid wall construction. I've been quoted £800 to tank 2 of the walls (this is just for the tanking...) which is outside my budget given that I need to remove all the render myself, and post-tanking dryline and insulate the walls, and then it needs a skim coat as well...

Question for the experts out there. I'm considering using damp proofing paint such as Permaseal or something similar. Is this a reasonable way forward and what is the best "damp proofing" paint to use (Permaseal, Cellarcote, Watertite, etc?) I appreciate this might only be seen as a temp fix but in the 11 years I've lived in the property it's the first sign of damp. Dehumidifier is currently sucking about 4 litres a day out which I'm guessing is from the render I need to remove...

Any advice welcomed.

 

No, there's no window around the damp area. There used to be a chimney breast where the problem area is, long since removed and no remnant of the stack remains. I've been into the loft space and the wall is bone dry so it's not coming down the wall. I don't know if the kitchen below is similarly affected as the wall is tiled and covered in kitchen units...

I know that the ideal solution would be to remove and replace the external render and treat the brickwork beneath, but I don't have anywhere near sufficent funds to do that (it's a big area...). Couple of guys that have been round to price it up have both recommended tanking inside as the solution, but the prices quoted for just applying the slurry are outside my budget as well.

 

Thanks for the link Phil. The render needs painting anyway (not been done since I moved in...). There are a few "cracks" apparent which may indicate some problems with the integrity but not around the area which corresponds to the damp. The render only extends as far down as the floor of the bedroom with exposed brick to ground level, and I'm wondering if the long-term solution might be to replace the render and extend all the way down. I'm only viewing the internal tanking as a short-term solution until I've got the funds to replace the render (may be a couple of years or so...) but am uncertain as the "best" internal paint solution to go for!