I see what you mean about the cap at the end of the bath. It is a shame, such a daft problem really.
The soil pipe is enclosed on the left hand wall. This was originally an outside wall but the house has now been extended on both floors so it is encased in a boxed in wall section within a new room.
So the lay out is soil pipe, wall, bath (lengthwise) then toilet. The back wall is outside but directly above an angled roof section with skylights. There will be a supporting wall/ lintel directly underneath too.
All in all nothing is accessible too easily (sorry!).
The Saniflo could do it but that leaves me with the challenge of running a vertical pipe from the toilet, and somehow breaking back in to the soil pipe from above. With the above info in mind, this sounds worse than moving the bath out. Even if I went for ripping the bath out, I may end up having to lift this higher than it is already, which won't please the kids.
Really sorry, it's not an easy one is it? I'm trying to think of the easiest and cheapest option, but also don't want to compromise the the aesthetics of what is otherwise a lovely bathroom.