Why does this happen EVERY time??

Question

I am disabled, a low phone user but use broadband a fair amount. Being on a fixed, low income I need to know what I'm paying out each month and keep it as low as possible. To that end I end up having a conversation each year with BT to get a decent deal.
Each year they appear to help me, on paper; I get reasonable quotes and go ahead with the contract. To make sure I keep my monthly payments down, I keep an eye on my billing online and make additional payments should I go over the usage allowance of my particular plan.
And then EVERY YEAR, they get the monthly payments wrong. Last year, out of the blue they took £44 per month when it should have been £34; I was refunded. This year, with a quoted monthly charge of £31-and-change, they have taken £37.50 and this despite me making - as usual, if it's required - an additional payment covering over usage on my last bill BEFORE the new contract was taken out.
I had asked for, and been given, a written total of the monthly charges.
My account page still shows "we will continue to take £34 on or just after the 3rd of the month".
And then they randomly take - as they did last year - a completely different sum altogether!
BT are in breach of the Direct Debit Guarantee by doing this; the rules say:
"If there are any changes to the amount, date or frequency of your Direct Debit the organisation will notify you (normally 10 working days) in advance of your account being debited or as otherwise agreed."
As (1) they have not notified me: they have clearly stated they will take a given amount and (2) this given amount is higher than the monthly charges anyway and (3) I have, as they suggest if you wish to avoid an increase, made an additional payment.... WHY ARE THEY DOING THIS?
I am tired of the constant battle with BT to just stick to what they agreed to charge me. The fact that I'm writing this at three in the morning should indicate the levels of stress this sort of thing can produce. I have emailed via the 'Contact Us' page on the website about this but no doubt I will get some scripted reply, if I'm lucky a small refund and then the same thing will happen AGAIN next year. I just want to know why this keeps happening!  

Answer 1

I am on the monthly payment plan because nobody at BT has told me there is any alternative!
Each year there I have a long, in-depth conversation with the person who sets up the contract for the next year but the gist of the thing is 'I'm  not going to be paying more than [x amount] per month, am I? Because I can't afford any more'' and each year the person says that no, in fact I should be paying less... and then without notice and even with the bill saying 'We will continue to take....' a totally different - and higher - sum disappears from my bank account.
Then I go through the painful process of writing to complain about this, get some sort of sum refunded and then the payments go through at the level they should, with me paying any additional (usually small) amounts for usage over the allowance. Until contract end when we start all over again...
Even if the amount they seem to randomly decide on is based on some sort of forecast, this should not be happening as I am doing what they instruct me to do on the bill to keep my payments the same.
So something is going wrong, on many levels.
At no point have I been offered the sort of 'monthly billing' described here; maybe there is some reason I am not allowed it. Unfortunately 'whole bill' direct debit payment is beyond me. I really need to know what I will be paying each month and I need BT to correctly take that amount, as agreed, each month, from the get-go.
It shouldn't be that hard really! Does anyone from BT read these forums? Can somebody take this on and finally get this straight, please?