Sorry folks ive been busy. Ok so there are probably many contributing factors as to why it went wrong in 8 months. Having said that i did prime the ply i did butter every tile meticulously, let the adhesive dry for 24 hrs as it advised, i squeezed and worked the grout very carefully into all the joints and sponged off leaving perfectly even grout line and let it dry for 24 hrs as is advised, i thought i had done good! it looked perfect. i still cant believe it failed so quickly.
I have now cut out all the ply up to head height on 3 sides, dwanged and fitted hardibacker, primed, taped joins, twice tanked, and laid new tiles on 100% bed. BUT im still nervous as on closer inspection the cheap B+Q tiles have varying amounts of glazed cover on the edge of the tiles varying from 0.5mm - 2mm! Secondly, i just noticed the bagged adhesive i purchased at B+Q was 3 month past its use by date. Thirdly, because im patching in i am held to the previous pattern and spacer size but i have noticed on the tile box it says "only use spacers of 3mm and >" i however had to use 2mm to match the existing! does that mean the grout dries to quick and loses integrity? should i seal/prime the open grout lines before grouting to slow the suction rate? I hate tiling......