Ianto

I stripped and replaced a slate roof back in the 80's you will need a minimum of 20% 'new'slates IMHO. To get a whole roof of reclaimed slate will be an expensive business I would go with the Spanish slate others have suggested it will be quicker and easier.

Having removed and reused slate when I reroofed my old cottage some 30 years ago I would suggest it is just not economic to pay someone to reuse slate either use a modern synthetic slate or go with Spannish slate. You will also need to check the weight of your replacement true Welsh slate was light about 19kg per sq m if I remember correctly you may need to add some reinforcement to the existing structure.