Thinking of paying monthly. Those that do it - how is it done, does the tax office give you a figure, does your accountant do it? is it started in April or just after your latest paymemt on account?
Much depends on whether you want to do it in advance or in arrears. For the latter, a bit of grovelling is sometimes required.
I am starting to get from a position of "in arrears" to "in advance". I should be making that leap during the next month. I intend to continue making my regular payment to HMRC even when my debt to them is cleared.
I do it by BACS. I think that BACS is probably the best option for people who have fluctuating incomes and fluctuating outgoings. That way, you can get further in front when you are flush and miss/reduce a payment if skint (not to be done if you are repaying debt). I actually arranged to pay them weekly and they were even quite happy to receive odd amounts so long as it added up to that weekly amount. At one point I was even paying my £100 a week as £20 a day and they were fine with that.
So I reckon BACS is best due to the flexibility. Maybe contact HMRC to ensure that you have their correct bank details and also make doubly sure that you set it up with the correct tax reference number. They will send you a statement through occasionally - usually when a half yearly payment is becoming due (or would be due if you hadn't already paid some instalments). You can start doing this at any point. They are a lot more flexible than they once were. Even if you don't pay in full like this, just top it up as soon as you know what you owe them. There will be some interest to pay if you are late with some of it. The rates are reasonable though and linked to base rate + something. It is my intention to have my July 31 bill paid by July 31 (for a change) by making advance instalments by BACS.