Being overcharged on plan, deal not being honoured.

In May I bought a 30GB plan at a reduced price of £2.50 pm for 3 months (normal price £10pm). The May payment was correctly charged at £2.50 but in June it was £5, I was on holiday at the time and did not query it then.
I recently received an email saying that the £5 was a mistake and it should have been full price?
Tried unsuccessfully to sort out through Live Chat and could not get the Contact Us form to work.
Now have further email advising that payment due 23rd July will be £10.
How do I get this corrected?
Thanks.

DavidC77 do you have a link to the offer you purchased? Ultimately you will need to pursue this with customer support if the discount is not been applied correctly.