I recently started a campaign using twitter ads. I logged in this morning to find that twitter have credited my account with £8, has anyone else had this?
No, that money must be from your credit card, Twitter has probably taken from there to pay for your campaign.
Could be that they reimbursed you for invalid / fraudulent clicks or something. I've never used Twitter ads, but that happens with AdWords all the time. You get credited for invalid / fraudulent clicks.
I think @qwikad.com is right. Twitter might be reimbursing invalid clicks, or views that came from users outside your set target audience. I don't think there's a way to withdraw that money, but you can put it into another Twitter Ads campaign.