Sker House is a thriller novel, written by Carnegie Medal winning author Ronald Welch, and published in 1955.
The story begins with Anthony Fitzgerald's waking up to discover he is chained to a sink in a factory bathroom somewhere in London, with no memory of how he ended up here.
Both the money and the necklace are now missing, and both men, after initially accusing the other of robbing them, realise that they must work together in order to discover who set them both up.
Going to the pub, they intimidate a frightened bar girl into admitting that she was paid to spike their drinks, under the mistaken belief that this was a practical joke being played upon the duo by their friends.
They force the guard at gunpoint, as he clings to the door, to tell them where he took the jewels, and he subsequently does (while sneering at Fitzgerald's foolishness), giving them a phone number to call for his contact in Bridgend, near where Anthony's boss lives.
The two men subsequently travel to Bridgend, and phone the number from a local phonebox, and, impersonating the guard, tell the man on the other end of the line that there is a problem.