Zavvi.com have the "PSN Live Card 20.00" [Note the incorrect naming of a PlayStation Network Card (not an Xbox LIVE Card) across all of The Hut Group's web sites] for £16.95.
"Please note: Your card will not be charged until your order is dispatched."
Potential QuidCo cashback (unconfirmed if valid for these items):
"2% for Electronics, Computing, Games Consoles
3.5% for Games"
PS. Unrelated to this offer, but worthy of note for all those wishing to purchase PlayStation Network downloaded games/content over this weekend, please note that the PlayStation Store will be unavailable between 8am & 9pm on this coming (Bank Holiday) Monday (30 August 2010) due to PlayStation Network maintenance:
Hence, make your purchases in advance of Monday. You *should* still be able to play online during the times specified (according to the European PlayStation Store Blog entry).