Dunno why it isn't working for you guys.
I just added a pair of full priced Tricker's to my basket and added the code hsback25.
It took off the 25%. I wasn't logged on.
I loged on and tried. It took 25% off then too even though I've already used the code.
I didn't complete checkout though.
Fine print from email:
*This offer is only for purchases made from 18th March to the 31st March 2014, on full price items. Minimum order value is £75 (£62.50 ex VAT). No other offers can be used in conjunction with this offer and is limited to one order per household.