Referral caused sub payment to increase not decrease

[#1]

My sister has been with Plusnet for almost three years, and has always paid £5.99 for value broadband.
I switched to Plusnet value broadband on her recommendation in August this year. I quoted her username on sign-up so she would get a 50p reduction on her monthly bill, i.e. down to £5.49.
Instead, Plusnet increased her regular bill to £6.49 and then applied the 50p referral discount, so she is still paying £5.99. My sister is mentally disabled and does not recall receiving any written notification that her regular subscription would increase. Her disability means she may have inadvertently overlooked written communications, however, I have not seen anything myself online to suggest that Plusnet have made subscription increases to existing customers.
Aside from the fact that the Direct Debit guarantee may have been broken here, can someone please clarify if it is possible that some customers are still paying a £5.99 sub for value broadband, or should they all have increased in line with the current new subscriber minimum of £6.49?

[#2]

Welcome to the forum
the change in prices was about a year ago, but existing customers had a years grace, you can find copies of e-mails sent out by PN in the ask questions section  https://www.plus.net/wizard/?p=search
they dont appear to have ones more than a year old though, this is from the 2nd of december 2010

Quote

We're writing to remind you that the government is increasing VAT to 20% from 4th January 2011. Back in March we promised that we'd freeze your broadband price for 12 months. We'll keep to our word, so what you pay for your FreeOnline Value service will stay the same. Champion.

[#3]

As pierre states, we initially increased the price of Value from £5.99 to £6.49 in April 2010. Customers already on the £5.99 variant were given a years notice of our intention to increase the price to match the normal price for and and we sent a reminder earlier this year as a reminder (this was sent around June this year and the price increased a month later).

[#4]

I joined on 18/8 and her billing date is the 9th, and she assures me the first time both changes were applied was on 9/10, which is later than you've said. It's in her favour, so no complaints there.  At least I understand now what's happened; i.e. it's something she should have been aware of and it's pure coincidence that her regular sub increased at the same time I joined and earned her a referral discount.
Thanks Chris (and Pierre). 
jim:quote

[#5]

mine went up one the 24th Aug and this is from the e-mail dated 21st June

Quote

Having lived up to the promise of freezing your broadband price at £5.99 a month for 12 months, we're now bringing your price in line with our current Plusnet Value service at £6.49 per month. Rest assured, we still offer the cheapest standalone broadband in the UK.^ On your next billing date after 12th July, you'll see your Value service charged at the new rate. Any referral discounts or other offers on your account won't change.