Form 64-8 is the most important piece of paperwork we need to claim your tax refund for you. This is the document that lets us act as your official "agent" with HMRC. Basically, it's what allows the taxman to talk to us instead of you dealing with him personally. You can download the 64-8 form here:

HMRC 64-8 Form - Authorising Your Agent

It's a lot less scary than it sounds, and means you'll never have to tackle the taxman on your own again.