Seems to be about the lowest price for this popular original HP black cartridge. £11.99 on the site then use promo code mad01 at checkout to receive a 5% discount = £11.39.
Go through QuidCo for a further 10% cashback on top bringing the price down to £10.25
Also known as
C6656A, No. 56, HP number 56, HP56, C6656AE, C6656
Going through QuidCo brings it down to £9.97 not £10.25 so even better value.