Caustic Soda down the drain
I read up on this stuff and is mostly suitable for kitchen drains where there's fat and food stuck. This stuff softens it up.
I made my own solution. I used my metal snake/ prodded to try and pull out all the hair, only a little bit came out.
I then blasted the power shower hose (Removed head for even more pressure) down the whole (Removed plug obv) and this bought up loads of dirt that swam around in the try where you stand. Absolutely loads of S*it.
I packed baking soda right down the pipe and added white vinegar then KABOOM the solution foamed up everywhere.
I then washed this all way with the power showers house and repeated the step again with Bi-carb soda and same reults.
I did this twice more with both mixes and rinsing the drain out each time.
It seems problem is fixed. It can take loads and loads of water and never gets flooded. I saved my self £80 as i didn't need a plumber.
P.s. Taking the pipes apart was my last option if the above didn't work. I didn't want to cut through the neat silicon that act's as a filler where the valance and the tiles meet and there's only about 6" of movement when you put your hands under the shower tray. All in all, took about 90 minutes.