What's lacking here is a good conspiracy theory, so here goes.
It's absolutely obvious what happened, and you can't blame the crew, but you can blame Crewe. Germany is repatriating its gold in response to gathering world turmoil, and the imminent collapse of the euro. (The repatriation is a fact, all kidding aside). So these are obviously gold Bentleys and Rolls-Royces, Goldfinger-style, made under conditions of vast secrecy in Crewe so as to get the gold they have in the Bank of England back to Germany un-noticed. They weigh 5,375 kg each, obviously unknown to the crew, and the result is there for all to see. The whole deal was put together when it looked as though the Swiss were going to vote to repatriate their gold, in the runup to the vote a month ago.
Because who owns Bentley and Rolls-Royce. VW!! QED.