The answer I hope is place the shower under the floor level. As I live in a "Walter Segal" house on legs this is not as crazy as it first sounds...I hope.
1. Remove rotten floor and joists
2. Except the loo! For now at least
3. Get an 1000lt IBC and mark it out
4. Modify the frame and weld to allow for new overflow and drainage
5. Dig out and level and try for size before cementing
6. Cemented in place and centre slab where drain trap will be is given a slight slope to ensure any leaks run away and dont sit
7. More to follow....