Thank you Gazza .
That's 4 - 4 now .
I'm going for .............. remove the tiles .
Only because I read my own Post and thought " Hang on .......!! Did I say 40 mm of cement ? "
Well yes I did . Plus the thickness of the existing tiles plus some adhesive plus the thickness of the new tiles !!!!
Must be nearly 2 metres deep now !!
That's maybe a decision made .