Who does Watson meet after his meeting with the postmaster?

After leaving the Postmaster, Watson meets Stapleton on the street.

“You will, I am sure, excuse my presumption, Dr. Watson,” said he as he came panting up to where I stood. “Here on the moor we are homely folk and do not wait for formal introductions. You may possibly have heard my name from our mutual friend, Mortimer. I am Stapleton, of Merripit House.”