Unauthorised Debit From My Bank Account After Nebula: Horoscope Cancellation

I started a £1 trial for Nebula Horoscope via the Apple App Store about four weeks ago. At the time of starting the trial, I immediately cancelled the subscription through my Apple ID settings, well within the trial period, so that I would not be charged further. Despite this, I have now seen a pending charge of £37.89 on my bank account, which I consider to be an unauthorised debit, as the subscription was already cancelled at the time of the trial. I request a full refund of the £37.89 and written confirmation that my Nebula Horoscope subscription is fully cancelled and that no further charges will be taken from my account.