Okay, so it's not exactly free but consider it a bonus to your shopping. When you spend £50 or more online at Dorothy Perkins you will score a free goodie bag worth over £20 and it's not a bad one either.
It contains:
Opaque tights
Avon Lip Radiance Gloss
Fiction novel
Essensuals £5 voucher 
Frusli Cereal Bar

With their Winter Sale happening at the moment (up to 50% off) that spare fifty quid could get you more than you realised.

Party Outfit for under £100 - Dorothy Perkins Edition
| Dorothy Perkins' Grey Pleated Wrap Dress | Dorothy Perkins 'Little Tee' Campaign: Sienna, Charlotte, Nicole and more