The Hut(.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 £17.93.
With the Bank Holiday Bargain "AUG40" (10% off a £40 spend) voucher code, you can purchase 3 x £20 worth of credit for 3 x £17.93 (£53.79) & reduce this by £4 to bring the total to £49.80 (!... yes; not £49.79) according to TheHut.com Checkout after "Your discount has been applied: -£3.99").
If you just want £40 worth of PlayStation Store credit; add two "PSN Live Card 20.00" (£17.93 each) to your Shopping Basket & then discount by £2 using the "AUG20" (£2 off a £20 spend) voucher code. Total purchase price: £33.86.
"Please note: Your card will not be charged until your order is dispatched."
Potential QuidCo cashback:
"2% for all purchases when a voucher code is used"
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).