Woodchopper;
Do you know if the dealer trying to get the part for you have checked the on-line parts locator (only available to Vauxhall dealers) to see if any other dealer in the country have the part you want in stock?
If another dealer has one Vauxhall will arrange for TNT to collect it and ship it to your dealer (who of course have to pay the other dealer for it!) There is no charge for the courier service- Vauxhall pay for it, as they are unable to supply the part themselves.
The cost of labour to repair a gearbox normally exceeds the cost of a reconditioned one, so most garages just replace them. Plus If a garage repairs a gearbox and it goes wrong again, shortly thereafter, because they failed to notice another part had been damaged when the gearbox failed they might be liable to cover the cost of further repairs. If they fit a reconditioned unit and it fails in under a year it has a warranty.
As others have said no car assembler is obliged to supply parts for any car they have supplied, even if it is under warranty. However under warranty they are obliged to fix it. So in your case, were the car under warranty, they would replace the gear-box.
As for the apparent disinterest, the person you spoke to may well know that Vauxhall have in stock reconditioned gearboxes, so you could get your car fixed (as others have suggested) by replacing the gearbox. Also, there is little they can do apart from tell you when it is supposed to arrive. If the factory that make it then decide not to make one (for you) because they have something more urgent to make....
In some ways you are lucky, I have been trying for almost two months get a delivery date for a cylinder head, let alone actually get it.