I've had a look online and Sainsbury's don't seem to do a Ready Brek type cereal (or at least where I live they don't)! They do seem to have instant porridge sachets though, which is what I hope you mean!
As long as they're just porridge oats and no flavourings, and weigh no more than 28g, you can have one as a part of one healthy extra B (but you must have either an oat or scan bran too).
Otherwise, 5 syns for 28g of porridge oats (dried). Flavoured sachets have various syns - just ask!