Taken from Clinton Cards' Facebook Wall and Twitter page this morning:
"ITUNES OFFER ALERT - hi all, we have another fantastic ITunes offer happening Friday 1st and Saturday 2nd April - £5 off £15 ITunes Cards. Whilst stocks last and t's & c's apply."
Possibly available online too - delivery approx. £3 (thanks to Nicster08 for info)
Edit: Deal is now available online with £3 delivery charge for upto 3 cards or £5.50 delivery for more - thanks to West and Welshman1982 for confirming
Unable to change the link to go direct to product, but discount shows when added to basket online - again, thanks to West for info.