Thanks for the replies.
Not entirely realising the time I went out around midnight

and had another look using my phones' torch.
When looking from the outside looking in, the angle actually illuminated the captive nut as still there.
My worry is that whoever fitted that self tapper either did so because somehow the nuts center is now too large for a tap, or that its destroyed the thread beyond repair. I did try to take a photo, forgetting that if you have the torch on, and try to take a photo requiring flash the whole phone instantly crashes - so I never got one
As the cost of rivnuts, metric bolts etc. came to about the same price as a tap, I've now ordered one and will hope for the best since maybe if it's of no use this time, it'll come in handy in the future.
Thanks to looking up about rivnuts, I found out that a special tool isn't entirely required, what you can do is use a bolt and nut of the correct size for the rivnut, and a nut that's too big and slides up and down the bolt. Use two spanners, one to hold the spacer nut, the other to turn the other which compresses the rivnut.
An alternative to the rivnut was the jack nut, this also is meant to have a proepr tool, but as long as you can hold the outer metal disc (the site I saw used vise-grips) while turning a bolt within it, this is enough the compress the jack nut and fix it in place.