I have just had an email come through for upcoming offers in O2. If you buy the Lumia 535 or 635 on pay and go you will receive a free portable speaker and £30 Microsoft voucher.
How to claim:
Buy the Lumia between 29/07/15 - 29/08/15
Go to Microsoft.com/mob…dle
Enter your full name, home and email address
Add the imei number
*#06# on the phone to get this
Microsoft will email you the £30 voucher and post the speaker
It suggest leaving upto 14 days after purchase for delivery.
The voucher can be used to purchase anything on Xbox, windows pc, or windows phone through their relative stores.