Welcome to the forum.
You could try applying the promo code manually to your account. Go to your My Account > My Passes page
and go go down to the Sky Sports Month Pass section. Click the apply a voucher link and enter PREM25EC then click continue and check you are being charged £25 before proceeding with the purchase.
Bear in mind if you already have a sports pass (day or week) on your account it won't let you buy a month pass until that day or week pass has been watched.
Also remember month passes renew automatically unless you cancel the pass before the renewal date (my advice would be to cancel as soon as you have purchased it, you are still able to watch for the remaining days once cancelled).
Hi @m4ttgtr take a screen print of payment page. If you are quoted a specific amount for a service, you can reasonably expect to pay that amount. What were the terms of the agreement? Did you get told how long the price applied for?
I got an Email about the PREM25 something offer, long since deleted, I didn't read the T+C's. I thought it was just for 1 month, at the start of the season.
Have you checked your payment details->order history? See if you made the £25 payment last month. I have a monthly movie pass running but the offer I was sent was accepted and will only be paid when it expires.