Charities Online
Gift Aid claims are not made by letter. HMRC requires them through Charities Online, and the account setup is a separate step from being recognised as a charity for tax.
Access requires a Government Gateway account linked to the charity’s HMRC record, with an authorised official nominated. The nomination is separate from being a trustee, and claims cannot be made until it is in place.
Set this up when you register for tax rather than when you first want to claim: see registering with HMRC. The HMRC charities helpline is free.