Cityseaedge

Hello, Despite a long career as a self-employed person, is my first year filing self-assessment without an accountant. I have already filed my return for 23/24. From my workings, I understood that I was due a repayment around £2,600. However, I've already received the repayment and it's around £1,900. Is there something I've not figured in to the repayment? Where can I find out how it's worked out? I simply worked out tax paid on account minus what is now due. There doesn't seem to be any error with the tax I've said is due. Many thanks in advance.