You can buy a kit off eBay for £35-40 to do that. Kit comes with primer which goes on first, mesh for corners and joints and big tub of liquid rubber type stuff which you put on with a paint brush - easy diy.
If you follow the above suggestion you will end up with more trouble than you started with.
1. What has drawn your attention to this corner?
2. Who suggested tanking?
3. Who suggested tanking for 2m?
4.Tanking in that corner might drive the damp sideways along the wall, & up above the tanking.
5. Is that a solid floor and a solid wall?
6. Is there render on the outside of the wall?
7. The key to dampness is to find the cause of the damp.
8. Do you have any problems with condensation?
9. Why do you ask about a receipt while asking: "how hard is it to do yourself"?
25 years
In a word: you are wasting your time & money until the outside issues are corrected. However:
Along the affected wall(s) you could remove all the accessible sodden insulation if its fibre glass(?).
Was the cavity clear when you removed the brick?
Difficult to see if you have a membrane under the slab?
You could hack off the plaster to 300mm above the outside ground level, or 300mm above the interior stains, & render up with 3:1 sand and cement and a remedial finish - this will give you some breathing space but its not a solution.
Another possibility is to tank with Vandex or similar but Vandex etc. could give condensation difficulties but it will hold the salts back for some time.
All damp co. guarantees are useless - the work is never guaranteed. Once they have your money it will be difficult getting them to answer the phone never mind returning year after year - a joke.
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