I'm afraid my previous attempts to remove the plastic have rendered an extractor useless. There isn't enough plastic left without risk biting into the threads.

I got a PM from another user about a used manifold. It's looking like my best bet.

 

I wouldn't replace the manifold over this... if nothing seems to work, take out the manifold and try again. You'll have more space, and then you have the liberty of working the other side and/or spraying fluids to help you get it out. A rethreading tap might be ok to use, but as said above, I wouldn't use a cutting tap. Maybe there is a barrel type tool you can insert, and tighten to expand the barrel, and then twist to expand its grip

Also try using an angled pick and tackle it thread by thread

 

You could also try burning it out with a low-temp torch, using a pick to remove the residue, and then chasing the threads clean afterwards. Might be easiest to focus the burn on one particular area so that you could collapse the remainder of the piece inwards after that part was removed.
I'd regard this approach as a real last resort though.

 

I've removed the guts of a broken plastic nipple with a pick as well. The plastic was pretty soft in my case so I just took my time and eventually got it all out without damaging the threads.

 

I used a extractor as well. and kept my vacuum sucking the whole time to keep contaminates going back into the block. then put a nipple on mine as well. I had jacked up my threads a bit and had to re tap them. I went a size up so I got a slightly larger nipple.