The sarcophagus on display in the Queen's Diamond Jubilee Galleries in Westminster Abbey.Photograph: Nils Jorgensen/Shutterstock

It has all the ingredients of a chilling mystery: the skeleton of an unknown man discovered within a Roman sarcophagus unearthed in the grounds of Westminster Abbey.

Now an expert has said he has solved the longstanding "who-is-it", suggesting the remains belonged to a medieval abbot.

First discovered in 1820, the limestone sarcophagus was officially excavated in 1869 from what is now an area of grass along the north side of the nave, but was once the yard of a sacristy built by Henry III. But the discovery spawned a mystery: whose remains were inside?