Advice on which Tiling, British Standard, to Purchase Please - Tile Advice

[Emma123]

Hi everyone, this is my first post so thank you for reading my question.
I am looking to purchase the British Standard for tiling but am a little unsure of which one I should purchase and as they cost so much money I am keen not to purchase the wrong one.
I would like it to cover workmanship for marble wall tiles, in a home shower area. I've been doing my research and I understand that BS 5385-1 is for normal internal conditions so I'm pretty sure I need to purchase BS 5385-4 but just needed to double check before I hit the pay button. Also in everyone's experience is it better to pay for the standard as is, or, to pay for the tracked changes standard (which is a lot more money). I'm not sure how often changes to the Standards occur. Thanks in advance for any help/advice received.

 

[Gillmoss]

Hello
had a bathroom done, they have used the dab method of plastering because never removed old tile cement,
Then they used 18mm marine strait on to the joists.
I believe this is not BS and never taped or sealed any joints.

the rawlplug showing gap in floor