I discovered something rather interesting. I did a shop and had a £10 off £70 voucher code so I used that. I went back into it a few days later and edited my shopping list - when I'd done, it came to quite a bit less than the £70.
When my shopping was delivered - I still had the £10 off.
I don't know if this was just a blip, or if you can always do it. But next time I do an online Tesco shop, I am going to try it with the £15 off £100. And then edit it down to what I actually want, and see if I still get the £15 off.