Recently elected as the patron of Paranormal Investigation, Mr Underwood spoke about three ghosts he had seen, and conducted a question and answer session.
Having written more than 40 books on the supernatural, he rates the South West as the most haunted area in the UK, and considered the Highwayman Inn 'the most unusual in the whole country'.
Michael Williams, co-ordinator of Paranormal Investigation, is requesting the help of Times readers to spot a travelling ghost.
He said: 'We are interested in the ghost of Lady Mary Howard travelling in her phantom coach from Tavistock to Okehampton Castle at night.
'Lady Mary died in 1871, so she's a truly historical ghost and we would like to hear from anyone who has had a sighting in recent times.
'The phantom coach travelled the route outside the Highwayman Inn, and Lady Mary is buried at Tavistock Church.'
Anyone who has sighted Lady Mary can contact Michael Williams on 01840 213401.