Cost of plan

I thought I would ask here first before going to customer service.
I have the 5GB plan for which I have just been charged £5, which is the usual pricing.
But when I set up this plan, I used a discount code that gave a price of £2.50 and I thought this price was for an indefinite period, not just the first month, even though I am on a monthly contract. Was I wrong about that?

Referral links get you a discount then go back to normal after certain amount of time.

Understood. But this was a discount code, not a referral link. I thought the terms said it was indefinite (I know some people have got an indefinite discount) but maybe I was wrong.
Not directly related to my issue, but I see that on the Money Saving Expert forum, there are plenty of complaints that a discount that should have lasted several months has not been applied after the first month.