It does not take a month to assemble and test a new PC - the assembly can be done by one person in two hours. In your position, I would be concerned - processors do fail, but a 'bad batch'? I don't think so.
If you were given 3 assurances that delivery would be before Christmas, and one of those was given prior to your order, it forms part of the contract. I would be phoning the supplier and asking for an absolute guarantee that this machine will arrive within the next two days. If that guarantee is not forthcoming, you might be better to cut your losses, cancel the contract, and buy another machine off the shelf at PC World/Currys.