wendyl - I've just looked at the Sainsbury's web site.
They have two lots of mulled wine displayed, both on special offer. They are listed as follows:
Sainsbury Mulled Wine 75cl ...................... . priced at £2.99
Sainsbury Mulled Wine, Taste the Difference ...... priced at £4.86
Is it possible you accidentally picked up a 'Taste the Difference' bottle? Maybe another customer picked up a dearer bottle, then saw the cheaper one, did a swap, leaving the more expensive bottle for you to pick up a little later!