NB: Tesco did a 2 for £2 a couple of weeks ago and might still be on. If it isn't still available, this is the cheapest out there.
I noticed this offer in my local Sainsburys petrol station, and it applied to all the 150g bags they had. Turns out it's also available online. Can't find it for cheaper anywhere else, looking at the online sites for supermarkets.
Nothing to go out of your way for, but they're my favourite crisps and are certainly worth a go at this price.
No end date given.