Hi Chris.
Horrible situation.
It would be nice to know that GasSafe was on your side and have the means of blocking anyone else's attempt to sign of this job, but I fear they can't and won't.
It would also be nice to think that all other GasSafe installers would smell a rat and not get involved in such a half-completed job, knowing that a fellow installer has been kicked off the contract for some unexplained reason. Again, tho', I fear they won't.
This might not be their 'fault', of course; expect the customer to lie through their teeth with their explanation (how incompetent you've been, how untrustworthy, how unhelpful, how rude, ok, I'll stop... )
So that basically means you have to tackle this issue the correct way, which is to make a small claims via your local CC, or online with MoneyClaim.org.
You need to ensure you do this correctly, tho', so get some good advice. You must demonstrate you've been reasonable and have fulfilled your side of the contract, up to the point they chucked you off with no valid explanation. This will require a hand-delivered (or recorded delivery) letter outlining your case and emphasising your willingness to continue to completion. You MUST be 100% professional at all times. Unemotive. Fact-based always.
If they don't see reason, then get the claim underway asap. If they live fairly close, it might also be worth driving by whenever you are in the area and seeing if there's another plumber outside - note down their number and give them a call, fill them in.
Good luck, and please keep us posted as this could help others.