Youngmaf is quite right, but if you want a typical cost...
Assuming it's only two storeys, and assuming there are no extensions in the way, the cheapest way is to find a local one-man roofer.
He'll put a ladder up from the pavement, hook a roof-ladder over the ridge, and shinny up there with a bucket of mortar.
£200 or so should do it. (More if he has to go off looking for replacement ridges if any are missing, or there are tiles/slates that need fixing.)
Unless hers was more involved, or she lives in the South-East, I think Margaret paid quite enough :o(