If you have a Waitrose nearby they operate a cycle trailer loan scheme- just register once at the store, and you get a Bike-Hod trailer that clips to your seatpost:-
http://www.bikehod.com/bike-hod/shopping.html
It takes plenty of shopping- enough for a week for most people I would think- and you keep it for three days. No charge. And if Waitrose is a bit more expensive than Grotco- well, it's a much nicer shopping environment, and think of the money you've already saved by not driving there!
Although I now do most of my shopping online- you can get practically anything delivered nowadays!!