Marks made by a stonemason more than 500 years ago have helped to date an ancient church tower in Somerset.

The carved "signatures" of a Wessex craftsman, which had been hidden since the stones were laid in the 15th century, were discovered during conservation work at St Michael's Tower on Glastonbury Tor.

Experts at the National Trust, which completed a £515,000 restoration of the famous landmark yesterday, found that the marks - two dashes - matched those found on a nearby church, St Cuthberts in Glastonbury, which has been accurately dated using documents.

Completion of the tower, part of a church built to replace a building destroyed by an earthquake, has now been dated to 1430.

Adrian Woodhall, the manager of the property, said: "We had previously thought that the tower dated from the early 14th century but when we began the conservation work we found these marks. Each mason had a particular symbol or signature to denote their work so that if it was found to be faulty they could be called back to rectify it."

The conservation work, partly funded by lottery money, was commissioned after repairs by the Ministry of Works in 1948 were found to be damaging the monument. The repair was carried out using a cement mortar which was damaging the stonework. The cement has been replaced with a lime mortar matching that used in the original building.

The restoration has been accompanied by work to repair the footpaths on the Tor, which was considered a spiritual locationbefore Christianity and is still a pagan landmark.

It also has a bloody history. A century after the tower was completed it was the scene of the execution of the Abbot of Glastonbury during the dissolution of the monasteries.