Someone complained about a similar situation to What Personal Computer magazine a while ago, they concluded that legally he had no leg to stand on...
The vendor had a delivery signature so the item was successfully delivered (the neighbour does not deny receiving the goods!) and it seems that the only legal course of action available to the consumer was to sue his neighbour!
As for the courier company, they do not have a contract with the consumer, but with the vendor. And in the case highlighted at WPC magazine, the contract clearly said that the courier is allowed to leave the package with a neighbour.
However the vendor did agree to settle with the consumer following the magazine's intervention, though the goods were only a mainboard and therefore relatively cheap. But the magazine concluded that they did not have too!
To be honest it would have been easier if the consumer denied receiving the package and also deny knowing the neighbour who signed the POD. But once the consumer acknowledged that the neighbour did indeed receive the goods, there is no going back.
So I think the only hope is if Dell agree to settle. Best option is to write to a magazine, such as 'Which?',