So, I finally got sufficiently annoyed by
Ocado arbitrarily deciding to stop me choosing whether or not to store the password for their website to write a trivial
Greasemonkey script to enable the functionality again.
You can get it here, if you would find it useful.
// ==UserScript==
// @name Enable Ocado password retention
// @namespace net.elvum.ocado
// @description Enables the retention of passwords for the Ocado website.
// @include
http://www.ocado.com/*// ==/UserScript==
var passwordBoxes = document.evaluate("//input[@type='password']", document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
for (var i = 0; i < passwordBoxes.snapshotLength; i++) {
passwordBoxes.snapshotItem(i).setAttribute("autocomplete","on");
}