The short answer
A mixture, as in most Cotswold-edge villages. The oldest buildings may carry stone slate or tile, later houses clay tile or slate, and the newest concrete tile. St Peter's itself was restored in 1864, so its roof is Victorian work on an earlier building. A survey identifies what each roof actually has.
Siddington sits just south of Cirencester, close to the Cotswolds National Landscape, and the village has about twenty listed buildings. A short walk covers a wide span of building ages, and the roofs reflect that. We never guess a covering from a photograph taken at a distance.
Q1.1What goes wrong with stone slate?
Where stone slate survives, each slate is hung over a batten by an oak peg driven through a hole drilled near its head. The roof is built in diminishing courses, so big stones sit low and small ones high. The pegs rot before the stones do, and slates then slip. The fix is usually to strip a slope, sort the stones and re-hang them on new battens, not to replace the covering. Our Cotswold stone roofing page goes into the method.
Q1.2What about clay tile, slate and concrete?
The village's brickmaking past makes clay tile a natural find. Slate travelled far once the railways spread, and concrete tile is the usual covering on later work, so a survey here may meet any of them. Each ages in its own way. Clay tiles lose their nibs and slip; slates fail at the nail; concrete tiles lose their surface and ridge mortar. An old church tower or a converted mill needs separate thinking again, because lead and masonry matter more than the covering.
The first visit is mostly looking: at the slope from the ground, then at the loft from inside, then close up with a ladder or from a safe vantage point. We keep photographs of anything that matters, and these go to you alongside the quote, showing the fault as we found it. Nothing is priced from a guess.