Discount not applied

I've finished an 18 months broadband contract with plusnet last month (I haven't received e a reminder to renew the broadband) so I called them on the 18th of December (2 days ago) saying that I was interested in a talk talk offer so they said they'd match up the offer and I agreed and paid for the annual landline saver but to my horror my monthly bill which was due a day after (19th) was changed to the full £34 for just broadband and phone which is more than the old and the new contract I negociated with them.... I called them again but they sait it was too late they've sent the direct debit order (which will be taken around 28th dec) and I was annoyed and told them that I was unhappy because this is the second time they breach the contract because last year they've increased the braodband by £1 so they said they can refund me the 12 pounds but the £34 stands.
Now I am really annoyed because there is no mention of discount on their website and I fear next month it would be another nasty surprise as it did happen to other people.
I spent a long frustrating time on the phone with them ...lots of time wasted.
The question I have is if I cancel the order within the 14 days how much can they charge me knowing knowing very well that they have broken the contract I agreed with them?
Plusnet doesn't feel like the same broadband provider I joined few years ago.

I'd say give them a chance to correct this on your next bill. Given the proximity of your billing date and your negotiating a new price it is quite possible that the increase was already in their system and too late to be altered to the lower negotiated price before the direct debit was raised.

what I find strange is that the amount increased overnight, it was less the day before.
If i could see the details of how much I'm paying monthly for just broadband (including discount) every month when i log onto my account that would make it easier but all they quote is the full amount without the discount

The DD would already have been in process, it usually takes longer than 1 day to change it. As long as they honour the price next month and refund you the difference for this month then that is what I would personally expect of them.

"18 months broadband contract"
Surely the contract is ongoing until you or the supplier cancel it so why would there be a reminder? The minimum term was 18 months.

If the cost is increased you can cancel if you wish.

Surely the contract is ongoing until you or the supplier cancel it so why would there be a reminder? The minimum term was 18 months.

At the end of the 18 months the contract would have reverted to a "standard" one month rolling contract and charged accordingly. As such it could be exited without penalty. The user should be informed of this but many suppliers, not just ISPs, often fail to do so.

Plusnet did exactly the same to me a couple of months back, such cockups have happened every time I have renewed my contract. The billing system was bad enough but after it was improved this year it's been even worse.

The long-suffering customer-facing staff will put it right in your next bill, it took them six weeks to sort mine but my billing is now back to normal and hopefully will stay like that for the next couple of years. I have always given PN full marks for BB performance, reliability and value for money, it's a great pity that their billing system is a continuing nightmare.

They reminded me when the annual phone saver was about to end so why not remind us when the broadband contract ended or is about to end?
On my new contract they actually mention sending an end of contract reminder.

" Account username: xxxxxxxxxxxx
We're sending this email to confirm the details of your new contract.
Your new fixed price contract
Great news, you're on a fixed price contract! That means the cost of your broadband and line rental won't go up while you're in contract. We'll send you an end of contract reminder, and offer an account review too, to make sure you're still on the right package for you"

It's important that they do because prices will increase automatically after the end of the contract

I contacted their billing service again this afternoon and they assured me that they will credit me the overcharged amount but it will be deducted from future monthly billings.
That seems to be more reasonable, so let's hope they get it right next month