Too late for your sunday roast, but here are the times for future reference:
Tesco Southwark is open til 10pm weekdays, 10pm Saturday, and 11am-5pm Sunday.
Tesco Kennington is open til 10pm weekdays, 10pm Saturday, and 10am-4pm on Sundays.
Tesco Elephant is open til 10pm weekdays, 10pm Saturday, and 11am-5pm Sunday.
(Goodness knows why Tesco are unable to publish open hours on their website. If you ring the 0845 number for each store, you can hear pre-recorded open hours. Strangely, the Tesco Storefinder doesn't list all stores' telephone numbers!)
If you need anything after 5 on a Sunday, the Tesco stores at Piccadilly and Goodge Street stay open til 6. (Piccadilly is only 10-15 minutes from Elephant on the number 12, and is useful when you notice youve forgotten something on a sunday evening!)
I also notice that several Sainsburys Locals in SE1 have restricted their open hours - they used to be open quite late.