With respect to the other posters...... it's essential to compare like with like.
Clay, "plain tiles"........ little ones approx 10.5" by 6"
Thousands of them. Time to lay them would be much longer than modern concrete tiles.
Leadwork around the chimney ....... not every semi has a chimney. This area is vulnerable and time must be spent on it.
Much more battening than other types of roof.
No link to next door therefore lead soakers up and down the parapet wall (important and time consuming)
Tile cutting on the hip takes time.
Proper clay "Rosemary" tiles are expensive. Are they replacing them, or re-nailing?
My guess is they are re-nailing. If a total replacement, then you have got a very good price indeed :o)
Lots of tiles will break. With plain tiles, lots will already be broken. They would have included for making up the shortfall.
Have I got it about right so far los? Before I think about figuring out the estimate?