how do those M & S chill bags work?

[hermykne]

its padded and snaps accross the top, do i put the bag into chill first and then put the food in and how long does it last for?

Do you mean the insulated cool bags. If so, you'll need to buy a couple of ice blocks (available from most supermarkets/camping shops) and freeze them overnight. Then just pack your picnic and pack the frozen ice blocks around it. Should keep your food cool for several hours.