How do I make a BBQ & a fire??

[MaryPoppinsPenguins]

I've got a folding leg firepit with a second grill piece to make it a BBQ.

I'm a complete camping novice and am going for the first time... what can I buy from a Sainsbury's / Homebase / local garage that will enable us to BBQ and then turn it into a fire for marshmallows?

Not sure I understand what you are after.

You can burn either logs or charcoal in a fire pit. You could use the top grill for your BBQ, then remove the top grill and re-ignite your fire with some kindling and sticks and small logs.

Marshmallows on sticks or skewers can be toasted on a charcoal or wood fire.