How: Order your M&S Christmas food online for in-store collection and you'll get a free £5 off voucher. There's no minimum spend, so you could buy the turkey butter baste for £1.49 to qualify for the e-voucher to use in-store or online.
Here's what to do:

Choose your Christmas food online at M&S* and arrange a collection time at your nearest M&S store. You can collect your Christmas order up to Sat 24 Dec, and it's up to Sat 31 Dec 2011 for new year.

You'll be emailed a confirmation note within 24 hours or ordering, which you need to take with you when you collect your food in-store.

The £5 voucher will be emailed to you by 30 November 2011. You can use it in-store or online within two years. There's no minimum spend.
Note, if your order is less than £20 then you pay the exact amount at checkout. If your order is over £20, then you'll pay a £20 deposit and the rest when you collect your order in-store.

The Detail: WARNING! If you don't collect your order within two hours of your allocated slot, it will be cancelled, but you won't get your deposit back. If you need to make any amendments to your order, visit the FAQ page for details.
hope you find this helps xx