Nerys,
If that bike doesn't work out for you, you might try checking out
AtomicZombie.com.
If you have access to a welder and basic tools you can build a bike or trike that will support your weight safely, for much much less than what the commercial units cost.
The best thing about building your own is that you can customize every aspect of it to fit
your individual needs, rather than having to try to fit to a "one size fits all" solution.
A quick example is this one I built for myself. The video was taken of my first "test ride", with just the basic running gear done.
I'm sure that if a dummy like me can do it, anyone can!