I did try one of those, but the section that goes into the waste needs to go quite a way in as its too small for the top, it onlly seems to grip at the bottom, but because of this the neck hits the rim of the existing waste.
The old toilet had a ceramic 90degree bend which went into the waste, this i believe is bigger.
Do I need some sort of connector or different pipe?
I think the only thing that would work in that situation is a cbo wc pan. It's more commonly used in Europe which means they're hard to get hold of and usually expensive.
The outlet points downwards as opposed to horizontally out of the back.
The only other thing I can think of is to maybe use a back to wall WC with a concealed cistern. You could extend the boxing from the bath to make a frame to mount your cistern in and hide the 90 degree waste pipe. Unfortunatlely this would render the pan you already have redundant.(maybe you could keep it if you plan to install an en-suite )