This roof was repaired and re-slated 25 years ago under the direction of R Emberton : Architect at a cost of about £6,000. The Diocese reports that it remains sound and weathertight today, in this very exposed and wet location. It is therefore already proving a very cost effective roof and can be expected to go for further decades without much attention.
It is best to employ your architect to carefully specify and inspect repair work to get the best out of the materials, for durability and value.
Rod Emberton is highly experienced in dealing with historic buildings and is on the approved list of architects to work on churches in the Diocese of Chester.