Can someone sort out my billing problem once and for all

[#1]

I am a new fibre customer having been with plusnet for years.  I signed up to fibre because you offered it for £5 a month for 6 months in your big advert on your site.  I did 2 online chats each promising me you would honour this deal.  But no.  Now I find I have paid £14.99 by direct debit.    What do I have to do to get the billing team to sort this out?

[#2]

Hi there,
The discounts were added after your last subscription payment went through, so they start in September for 6 months. They are definitely correct and in place from then.