Are we allowed to drink these?” asks Terry Pratchett, pointing at the large green drink with chillies and stalks of mint sticking out of the top which is sitting on the table in front of him. It is 11.30 in the morning, and I am drinking a chilli mojito with one of Britain’s bestselling novelists. Pratchett prods his, inquisitively. “Do I have to take the knobbly bits out of the top?”