i had the same problem (well until yesterday) Got cilit bang poured a good bit down the drain as much as I could untill it filled to top, then waited added more. I kept repeating this until half the bottle was gone in. Poured some hot water down next and left it. Next morning, had a shower and water flowed FREELY AWAY.
I was amazed, for past few weeks, me and himself have been up to our ankles in water!!
So refreshing to have a clean shower. I also got some cif spray bathroom multi prupose thing for shower doors too, like for the lime and all. Worked a treat can actually see through them now.
Our house is only 9months old. I think our shower prob was down to my long hairs, lifted the plug thing and got loads.
Hope ive been of some help.
Blueshoes