Asda-Entertainment.co.uk 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 £17.91.
With the Bank Holiday Bargain "BH4" (10% off a £40 spend) voucher code, you can purchase 3 x £20 worth of credit for 3 x £17.91 (£53.73) & reduce this by £4 to bring the total to £49.73.
If you just want £40 worth of PlayStation Store credit; add two "PSN Live Card 20.00" (£17.91 each) to your Shopping Basket & then discount by £2 using the "BH2" (£2 off a £20 spend) voucher code. Total purchase price: £33.82.
"Please note: Your card will not be charged until your order is dispatched."
Potential QuidCo cashback (unconfirmed if valid for these items):
"8% for Asda Direct, Entertainment (cd's, dvd's, books and games), Pharmacy, Sunglasses, Gifts, Babyshop, Electrical, Flowers, Furniture, Homeware, Motor, DIY, Optical, Pharmacy, Photo, Sports and Leisure, Toys, Travel (excluding flights), PAYG Mobiles and Accessories"
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).