Preparing shower recess for tiling

[JamesA]

I'm currently refitting our main bathroom, ensuite and downstairs toilet.
I'm going to be doing the preperation and tiling work and a plumber is going to install the bathroom and shower.

We have a 750ish square recess where the mixer shower goes.
I've stripped out the old shower down to plasterboard.
The old shower tray was lifted up about 8 inches on a riser.
Where the tray and tiles met, a line of black mildew/mould has been left:
http://www.photobox.co.uk/album/album_fullsize.html?c_photo=79862856
http://www.photobox.co.uk/album/album_fullsize.html?c_photo=79863239
The back wall had foil-backed plasterboard that I've removed as the water had got to it and it was crumbling. It was attached to the plasterboard behind it with tile adhesive and plasterboard nails.
The backwall of plasterboard you can see is the back of the airing cupboard.
Water has penetrated this plasterboard at the bottom as the layer I removed didn't go down any further than the tray.

It is usual/ok to have the shower plasterboard backed up against another room's plasterboard in this way?
I'm thinking of adding a layer of Wick's aquapanel to the whole of the back wall.
I was also think of replacing the damp-affected bottom of the side walls with aquapanel rather than plasterboard, but will there be a problem with the aquapanel being slightly thicker?
Close ups of mould:
http://www.photobox.co.uk/album/album_fullsize.html?c_photo=79863240
http://www.photobox.co.uk/album/album_fullsize.html?c_photo=79863241

I've bought a BAL WP1 shower tanking kit for the places where I'm leaving plasterboard.
The paper outer layer of the plasterboard has come of when the old tiles were removed.
This has left it brown and flakey:
http://www.photobox.co.uk/album/album_fullsize.html?c_photo=79863249
How can this be best sealed before apply the WP1?

Shower floor:
http://www.photobox.co.uk/album/album_fullsize.html?c_photo=79863250

 

[JamesA]

I don't know what happend to the photos:
http://www.photobox.co.uk/album/4017964

 

[Nige F]

Definitely go for the Aquapanel backing in place of plasterboard .........I`d rip the lot out and do it all again ..........also where there is remains of adhesive on ANY substrate it makes tiling difficult ..for me anyways..........and sort out a fan for ventilation hth.

 

[JamesA]

I'm going to rebuild the 3 walls again with the aquapanel I bought.
I'm going to leave the airing cupboard plasterboard at the back as it is.

The trays seem to come in a standard 760x760 size.
The previous tray was actually 750x750.

The recess is actually currently 745.
It will be easier to leave it the current width.
Should I make the recess exactly 760, or should it be a little smaller to allow the plumber to cut in the tray?

It will be a Mira Flight tray with riser kit with 3 upstands.
Width 760, width including upstands 770.