For anyone who may be interested, I have managed to find a supply of
reclaimed tiles to match my existing roof. They come from a company
called Clydesdale roofing supplies who seem to have a number of
outlets around the north-west. They normally deal in new materials
but seem to keep a small stock of reclaimed tiles. Unfortunately I am
having to pay a little more than I would for new tiles (£1 +vat per
tile) but having rung around a few places, everyone agrees this is a
realistic price given the difficulty finding them.

Thanks for all the help/advice.