I've just replaced mine, got it from GSF £90 odd pound, doddle to fit.
You really need to Hammerite the new filler pipe first, then make sure you clean under the wheel arch really well, then apply some Waxoyl or other underbody paint around the inside of the rear wing then fit the new filler neck.
Should last for years :-)
Otherwise, you can buy the tin top version of the filler neck. Its the same shape, but its just a bit to long, which means you can cut it to the right length with a hacksaw.
The only stumbling block is the bracket that is welded onto the side of the filler neck. Its at the wrong angle for the convertible, and needs to be cut/removed, then welded back on in the same position as the old filler neck.
If you know someone who welds, then buy the tin top version, modify it and save yourself a few quid.