Recently had a builder on our roof to do some small work around the chimney. He said that the tiles were in a bad condition, rosemary clay, and could do with replacing. The tiles are around 70 years old and the roof has never leaked. He said that although the tiles look fine from the outside they have started to seperate between the layers. I would have thought that they would have weathered from the outside first. We have a semi-detached place and he quoted just under £8k to felt the roof and then re-tile with new clay tiles.
Does all this sound kosher or am I being conned?

If your roof isn't leaking and everything looks sound and dry within the loft then you don't need any work done... sometimes rosemary tiles will break in half across the width leaving the gap between the underlying tiles exposed causing an ingress of water in harsh weather, however, rosmary tiles can be easily lifted and replaced individually and are very cheap to obtain from reclaim yards.