Home DIY

...Gypsum based plaster ... don't use bonding though as it is very absorbent and shouldn't be used on exterior walls especially next to windows, or in shower enclosures- use cement render (with a multifinish skim if you want). If the repaired substrate is dry, flat and plumb go ahead and tile on top.
.... or you can just use good tile adhesive (I always use Mapei and not just cause of the cycling connection) The adhesive will be flexible, dry faster/take tiles sooner but costs more. ...

Thanks absurdbird!!

I followed B's advice earlier in this thread and I've now taken out the wooden sill – which most definitely was the culprit – and poured waterproof cement into the cavity (with a bit of plywood as a scaffolding).
It now looks like this:

But as i struggled a bit with the waterproof cement (much stickier than normal cement I reckon) I ended up with a sill that is not quite level, it's a few mill off to the left side. This is where I need to properly understand what you are saying about plaster and stuff.
I've started trying to make the sill level with some Thistle 'one coat plaster'. So is that right by you or do I need some other kind of plaster ? I am getting really confused by all the different brands of plaster.

Eventually I will tank the whole shower section with the BAL shower kit then tile all over. Will the BAL tanking kit be good enough to cover whichever plaster I use to level the sill? Or are you actually saying I should use the BAL waterresistant adhesive to level the shelf? If so do I then tank on top of the adhesive again?

Btw, building is a block of flats built in 1990.