John Newton – Amazing Grace (tour – Cambridge)

Saltmine is a Christian theatre company. Its latest touring production is of a new play scripted by Richard Hasnip based on the life of the late 18th century sailor, poet and clergyman John Newton. His time at sea included being press-ganged into the navy and serving on a slave ship taking part in the infamous "middle passage" between Africa and the Americas.

The first half of Amazing Grace ends with Newton escaping being tossed overboard by the slaver's master as a "Jonah" through his bravery when the ship all-but foundered during a storm. As men do in such circumstances, he prayed; unlike many who escape imminent death, he retained his new-found faith and passionately wanted to share it.

His wife Mary (known as Polly) was supportive through this change of professional direction and her death from consumption hit him hard. He appears to have been a charismatic preacher to his congregations (once he had secured ordination from a somewhat sceptical Anglican establishment) first in Olney and later in London's poor Woolnoth district.

Poetry was one means by which Newton spread the message of both the Gospel and abolition. He had friendly relations with Cowper and with Wilberforce and hi best-known hymn "Amazing Grace" has had an extraordinary after-life – including being the all-but official anthem for the US Civil Rights movement as well as a staple of the popular repertoire.

Thomas Firth's production is set on the deck of an 18th century ship with a suggestion of sails on three sides. There are five actors – I presume from the programme that it has been double cast – but it gives no indication of who actually plays who. Costuming is vaguely of the period and there's an ecellent use of sea-shanties and folk-songs.

The difficulty for any sort of committed theatre lies in making the characters we meet credible, rounded human-beings and not just types. This works better in the first half with its constant flow of action and shifts in location. The second half – I won't say suffers, but… sags a bit under the weight of Newton's conversion. It is perhaps too speech-heavy.

John Newton – Amazing Grace is on national tour until March 2015.