It really doesn't look like it'll come out from underneath. I've undone the engine mount, raised the engine etc. to no avail. The car is up on stands and the downpipe doesn't hit the floor- it looks like there's not enough room between the chassis and gearbox for it to fit out the bottom.
I've given up for the time being. What a ridiculous setup.
I think it'll have to come out the top, which means removing the manifold... Much easier said than done, obviously the person before me had as much difficulty in finding a suitable tool and has badly damaged the manifold nuts...
I'm seriously thinking I'll retreat and put the old one back on.