Just been in my local Tescos and they're selling a pack of 2 foil cooking bags for just 1p! Thats right, 2 for only 1p!
They are 28 x 42 cm and can be used in the oven or BBQ for cooking meat, fish or vegetables. The food is steamed at the same time as it's being roasted so it retains it's juices and doesn't end up dry.
Usually cost around £2 or so.