As per reply from Julian Sidney, decoupling essential. I have just finished a floor using 20mm limestone in 4 sizes from 400x400 to 1200x800. 28 sq m, tiles laid in 2 days, but decoupling (priced separately), wash, pre grout seal, grout, final seal, had to spread across 6 days. I priced this at £30 psm because builder gave me labourer to help (1200x800 and 80x800 need two to lay flat) at his expense. Otherwise price would have been £40 Psm.