I love that show and it's still relevant today.
The US Ambassador is the plum job in the diplomatic service and usually goes to a long serving diplomat whose paid he's dues in far away unglamorous places, for many years (s*** holes if you like). So it's often their last job before retiring as a thank you. These days it's not nearly as important as leaders just pick up the phone and talk to each other.
So I would have thought that when the PM nominated Mandelson the Foreign Office Civil Servants would be miffed it wasn't one of their own and would happily undermine the process. The only reason I can think of why this Foreign Office official overrode the failed vetting is that someone told him to.
When Harold Wilson appointed his son in law (TV Pundit Peter Jay) to the same post there was uproar in the FO at the time as he wasn't one of them.
That's why it stinks, someone is not being truthful here.