Hmm; I’ve finally got on the site; this is the blurb for home delivery:
How late can I make amends to an order?
Every order has a cut-off time, but it can vary depending on when you placed your order and what delivery slot you booked. Please check your confirmation email to find your order's cut-off time.
To allow us to deal with unprecedented levels of demand, please finalise your order as soon as possible after placing it, and several days in advance of your delivery - no later than 72 hours before it is due to arrive. Due to unprecedented demand, further edits after this point will cause disruptions to our fulfilment service and could cause your order to be cancelled altogether.
UPDATED ON: 31 MARCH 2020
Vs the blurb on click and collect:
What if I want to edit or cancel my order?
Once your order is placed in advance, you will have until 11:55pm the night before the day of collection to edit or cancel your order. After this point, please contact the Customer Service Desk using our 'Contact Us' form.
UPDATED ON: 27 MARCH 2020
Looking at the updated dates, it does look like it’s been amended during the current situation but click and collect is still the night before.