Misleading Promotion and Unexpected Charges

I signed up after seeing a promotion that looked worthwhile. After receiving the quote, I noticed additional charges and queried them. I was told it was a once-off fee, which seemed fine—until I received the mandate. The amount had doubled, with an added "Promotion Fee" included.

When I enquired further, I was told the billing was pro-rata, so I'd effectively be paying for two months at the end of this month. I then asked if the "Promotion Fee" meant I was actually paying the normal price, and the response confirmed that I was.

How is it acceptable to advertise a promotion, only to charge customers the full price anyway—just under a different label? This feels very misleading and not what was initially promised.