Wasn't sure whether to put this as a deal, voucher, or discussion.
For those with a Halfords Trade Card, you can go to the link and login to your account and add a trade card number. This gives you the trade card prices online, which can be useful as it is less hassle than going to the store and asking them to scan for you.
Maybe everyone else but me knows, though I wasn't aware of this until I was told instore today.
Additional savings can be had using TCB for 3.03% (thanks smegal). Also if you have a Love2shop/Flexecash card (I was given one by my workplace) you can top up, and collect instore, you can use that for an extra 7.5% off. Note that you do not need to present the trade card instore for the discount.