Avoid Tesco at all costs. Even if you do get a contract, after a few months they will demand that you cut your prices. And then every few months later they will demand that you cut your prices again.
You will be tied into a contract that is seemingly binding on you to provide the service but that they can do anything they want - and the only rule is that you can't win.
Your work will be subjected to a RAG quality audit Red, Amber, Green. As long as everything is Green, you're OK. If anything happens to go Amber or (heaven forbid) Red, your life will be an absolute misery.
Avoid Tesco like the plague.