Market 3 area but price has increased after 3 months

[#1]

Hi guys,
I hope this is the right area for this. I ordered and received my premium broadband package in March at the cost of £11.99 a month. I checked my market area before ordering to make sure I was in a low cost area but unfortunately I am now being charged £15.99.
Is this simply a mistake or have any details changed that I am not aware of? And if it is an error, who would I need to speak to to help resolve this?
My exchange is Bangor, Northern Ireland and I've copied the details of the exchange that were provided by the plusnet usertools :
Exchange Status Checker
Results: Bangor (ni)

Code: NIBA
County: Northern Ireland
Enabled: 29 Jun 2000
Market: 3
Colour: Green VP capacity at this exchange is currently showing as Green.
Click here for further details
Colour: Green BT have advised that this exchange is enabled for WBC on 21CN
Colour: Unknown You can view recent Service Outages for this exchange here
Colour: Unknown BT has published DSL Max upgrade information for this exchange
Thanks in advance

[#2]

Hi Mikaay,
I'm sorry if there's been any confusion here, but you are being charged the correct amount.
Plusnet Premium starts at £11.99 for the first three months and then moves upto £15.99.
If you were in Market 1 or 2, it would have risen to £19.99.

[#3]

Hi Jamesseh,
Thanks for the clarification! That's no problem at all.
Thank you for the speedy reply

[#4]

Hi Mikaay,
That's no problem at all.
One thing to consider though, if your usage is likely to keep under 60GB per month, it might be worth having a look at our new Extra package, which would be £11.49 ongoing, so you'd save yourself £4.50 a month.
You can change your product once a month, but if you move away from Premium, because it's a legacy account, you wouldn't be able to move back to it.