Update : It looks like this one is unlimited and you can order more than once!
Move quickly if you have a Sparks Card from M&S as you could grab three pairs of tights completely free!
First you have to log into your Sparks Card* account and add the promotion for "£5 off women's workwear" It states this excludes tights but currently doesn't appear to. Since three pairs are £5, they're going through FREE.
Move quickly though as these will go fast! You can check out the whole workwear range here*