They have a museum, and it has a website, on which interested persons can find a good capsule history of the church:

http://www.engleseabrook-museum.org.uk/history.asp

The best collection of PM materials is at the Archives of Methodism in the John Rylands Library in Manchester. I have been there to compare different editions of their early hymn book. When it only contained sixty hymns, over a third were from the American camp-meeting movement; sixteen were by one rural preacher from Tennessee. This lapse of poetic taste was corrected fairly soon, mainly by Hugh Bourne; but the original hymn collection (brought over by Lorenzo Dow at the end of 1805) was informally kept in print by Dow's admirers in Ireland -- not necessarily affiliated with the Primitive Methodist Connection -- at least into the late 1820s.