How easy is it to get a refund from Halfords?

Our car battery died this afternoon so my husband went to Halfords to buy a car battery charger. He opened the box but before he used the charger, he thought he will give the car another go as the battery may have become useable again after the hour or so since it died.

Lo and behold, the car started to our relief!

We want to return the battery charger to Halfords as we don't need it and it has not been used although the box has been opened.

Anyone know if Halfords are ok with returns or are they sticklers and will say no because the box has been opened?