If I may, fellas, I'll go in reverse order with the replies since my memory is dire.
Stuart: Unfortunately, this particular barrel's plug is resolutely committed to staying precisely where it is; having got some rust and dirt off it, I'd surmise it's been hammered in while both it and the barrel were hot, the whole then being left to shrink-fit together. This is a right royal pain in the backside, quite honestly, since with a threaded plug I could simply remove it, get a torch and have a good nosey around within. Why these Indo-Persian gunsmiths were so keen on sealing their breeches this permanently I'm honestly not sure; it makes cleaning them out an utter bind and makes very problematic an obstruction which, in a threaded-ended barrel, would be a minor annoyance.
Curiously, as an aside, this barrel looks to have seen some use, yet the touch-hole is positioned what seems very far back along the barrel; so much so, in fact, that I think I can see the breech plug through it, forming a wall that covers the rear third of the hole and forces the gas jet from the priming pan through a turn of about 60 degrees. It's almost as if the barrel was made without too much reference to where the lock would be in relation to it, and the hole drilled there out of necessity.
Anyway, Philip. Amazingly, you haven't yet screamed in horror at the awful clunker I've acquired, which is nice! Regarding the Immovable Screw of the Kandahar, it turned out to be less immovable than I thought; leaving WD40 to soak into the threads for a bit, followed by a larger pair of pliers than I had at home, did the trick nicely, and revealed that the thread on its tip is barely cut at all; it seems that what's been holding it in place for so long is rust. Since the gun isn't going to shoot any time soon I've left the screw reasonably slack, seeing little need for absolute firmness in the lock. It's in place, but not particularly tight. All this at the cost of a few small gouges and scratches in the wood, so I'm quite pleased really; a small price to pay for making the Immovable Screw of the Kandahar become the Not-Quite-Immovable Screw of York
Regarding the questions you asked: the bore is, roughly,. .615in, or a 20-bore, give or take, or so the dealer averred. Measuring at the muzzle with a ruler - I have no means of measuring down the barrel - I get 5/16 of an inch, which about matches up. I have here a pair of cleaning brushes, both of which seem to be rather oversized, but one of which has been down the works jezail and thus become quite well shaped to that barrel which, although rifled, is happily of almost the same bore. I do indeed have a worm, but it's attached to a piece of wood and really only good for breaking up compacted powder etc; I'd have to nip to York Guns, probably, to get a worm/auger attachment, and getting it in 20-bore might not be too easy, but I shall have a go.
And Battara, thanks! I shall greatly enjoy working on my first antique gun!