Painted by the local artist John Millar Nicholson, the wall paintings in St Thomas' Church are a magnificent example of late 19th century monumental wall painting, and have no surviving paralell on the Isle of Man. The slate-like Manx stone, from which the church is built, is notoriously vulnerable to penetration of rainwater and, as a result of this and the failure of the rainwater disposal system, areas of the wall paintings had suffered severe and extensive deterioration.

Tobit Curteis undertook a detailed diagnostic study to identify the precise causes of deterioration and to work with the PCC to control them. When the building was in a stable condition the treatment of the wall paintings was carried out in a number of phases over a period of two years . As well as the stabilisation of the paint and plaster layers, large areas of reconstruction were undertaken on sections of the paintings which had been entirely lost.