Update - just tried the link again and struck lucky!
They were only offering delivery for one day - Monday 30th March. So, I took the slot and ordered one for MIL, that will take the pressure off a bit, as she is self isolating due to being vulnerable.
We do get shopping for her, but we are both have sore throats, so are very conscious we need to keep a distance from her (and everyone else) just in case. We are trying to only shop once a week, as the less contact we have with others, the better, but it is tricky as you can't always get everything you need, when shelves are empty.
MIL is very good at making basic supplies go a long way, and not a fussy eater, so this will ensure she has the basics, in case we can't get over to her.