Go to 'Promotions' tab and enter user name and the following codes to get free credit:
(First one is today)
Mon 8th June - 252250602 (£1)
Sun 14th June - 252250603
Mon 15th June - 252250604
Fri 19th June - 252250605
Sun 28th June - 252250606
Don't forget to try the £5 voucher code (WCP-2734158-5) as it worked for me although I have yet to see it credited to my account.
Credit can take up to 72 hours to be credited but seem to be done within 24hrs in my experience.