I've had to submit a self-assessment this year for the first time and the amount due is way higher than expected, mainly due to my car benefit from work (partly because it was wrongly calculated originally so I've been underpaying all year but also because it's included as income for high earner child benefit charge purposes so I also have to pay back more than I thought of that). I don't have the money to pay it as a lump sum so will need to arrange a payment plan but I can't find much info about how they calculate the installments without just going ahead and setting it up, which I can't do today as I need to wait until the final calculation is in the system.
Does anyone know how much interest they add, how long I can pay it over, etc? Do they tell you a monthly amount or do I choose one? Ideally I'd like to be able to play around with different options of initial payment and monthly payments to see the overall cost but I have a feeling I won't be able to.