A roof at Heacham and the same roof at Swaffham are not the same job, and pricing them as though they are is why some coastal roofs need attention again inside ten years.
Wind uplift is about the edges
Wind does not press evenly on a roof. It accelerates over the ridge and around the gables and creates suction at the perimeter, which is why damage almost always starts at the verge or the ridge and works inwards. On the coast that means clipping every tile in the perimeter zones rather than relying on the tile hanging on its nib.
Mortar is the wrong material for a bedded ridge here
Mortar-bedded ridge and verge relies on a rigid bond between two things that move at different rates. It cracks, then it lets go, usually in one piece and usually in a gale. Dry systems fix mechanically into the tile and the batten, and they do not care about the weather.
Salt and fixings
Galvanised nails that would give you thirty years inland corrode noticeably faster in salt air. Stainless costs a little more per roof and removes the problem entirely. If somebody is quoting you for a coastal re-roof and stainless is not mentioned anywhere, ask.
The honest version
None of this is exotic and none of it adds much to a re-roof. It is the difference between a roof that lasts as long as it should and one that costs you a repair every other winter.
