Overcharged bill

[#1]

Signed up for Plusnet's unlimited broadband 12- month contract.
Got my second month's bill and it was more than my first month's. I was told I would only be charged £15.75 every month, but I just received an email from plusnet saying
"We'll soon be taking a payment of ｣18.45 for your service. This will leave
your account on or after 13/11/2014.
This email is sent for your information only, you don't need to do anything
to complete the payment."
Where are the extra charges coming from?
Can someone explain to me why I'm facing these extra charges, or point me in the right direction.
The only reason I considered plusnet was its cost
1 Unlimited (Contracted) - Market 2/3 charge 10/11/2014 09/12/2014 £9.99
1 Weekends charge                                         10/11/2014 09/12/2014 £15.95
1 Call Plan charge                                         10/11/2014 09/12/2014 £0.00
1 Offer Code Discount - augoffer14                           -                -          -£7.49
I cannot find a clear reason for this. The breakdown of costs states that I am paying for "Plusnet Unlimited (Contracted) - Market 2/3 charge". I see this as a bit of obvious obfuscation on the part of Plusnet. What is a Market 2/3 Charge? None of this was ever made abundantly clear, or clear at all, upon signing up. As far as I can see this is a clear case of 'bait and switch', because I was promised a cheaper deal (the only reason I ever considered plusnet in the first place) to be hosed by higher costs. Burying caveats in walls of text, and using faux legalese to do so, seems fairly morally reprehensible if it turns out that this bizarre market charge was mentioned in the supporting documentation.

[#2]

The alternative to the Market 2/3 charge is Market 1 which is £7.50 a month more!

[#3]

Hi there,
The bill breaks down as follows:
1   Unlimited (Contracted) - Market 2/3 charge   10/11/2014   09/12/2014   £9.99 - This is the cost of your broadband service before any discounts or special offers are applied.
1   Weekends charge                                           10/11/2014   09/12/2014   £15.95 - Line rental charge
1   Call Plan charge                                           10/11/2014   09/12/2014   £0.00 - If you had a call package it would show here
1   Offer Code Discount - augoffer14                              -                 -             -£7.49 - The discount on your bill, this is due to the offer you signed up to. Discounting by £7.49 means your broadband charge is £2.50 per month as agreed.
Your first month's bill was slightly lower as we pro-rata the line rental, so you only pay for the period from when it goes active. The ongoing amount will be the same as this month's charge.

[#4]

If you went for the 'Line Rental Saver' option then the cost is £155.88 for a year's line rental in advance (equivalent to £12.99 per month), then you'd be closer to the £15.75 that you mention.  Do note this also means you no longer get weekend calls included with the basic line rental.