I can tell you what happened to me last year, without doing anything proactive. Like you, I knew my monthly payments were too high, but, in August, the amount taken suddenly decreased and then I had no payments at all taken in September and October. I think by August, they have calculated how much tax you actually owe for the current year and if your monthly payments are going to result in an overpayment, they don't take any more once your tax bill is paid. My payment in August completed the total amount due.
Hope that helps.
Not sure about monthly payments coops but you can vary it with a quarterly one by going into your account on the tax payment website.
I increased my on account payments last year as a hedge against an increase tax bill as I thought the bill would be higher than the previous one, what a difference a year makes.
However, if you cannot change the monthly amount, and the total you owe is less than you have paid you will get a credit to your bank as soon as the tax advices come out.
Why not post a sensible answer, people will appreciate it more
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