Pulling up this thread again, I've just started waterproofing our wet rooms. We bought the AKW formers from a company called Hurlston on eBay. They were very good value and excellent quality. They are 22mm the same as the flooring. The floors we covered in 12mm marine ply so we cut the holes for the formers and re inforced underneath as per @Nickfromwales instructions and then filled these with the ply to bring them up to the same level as the finished floor, then I glued and screwed everything down.
I was pretty concerned about doing all this we haven't done anything like it before. The tanking of the rooms has not been difficult, I just watched the YouTube videos and followed instructions. A bit of a messy job but very quick and easy.
So really just wanted to reassure anyone considering doing this for the first time that it's a pretty straightforward job.
I'm just hoping our tiler is happy with the result!