I think if you go in and put it like you have put it here, you'll get a sympathetic ear! If you don't SW will probably find out anyway as they ask employers (every 13 weeks or so, I think) to certify what days a claimant worked and compare them to the signing record. And what you did is more common than you think; it's too tempting for some people to 'forget' that they worked a day and sign for it instead! That is why they do checks and comparisons.
If you voluntarily go in to sort out, they will appreciate that. Maybe employer can give you a record of the days you wroked? You donlp;t have to say why you need it, just say SW want it or something. Go in with this record and tell them that you think/know you didn't sign for correct dates, asking them to check it out with the employer's record. They should come back with a figure that was overpaid to you and then you can repay that amount in a lump sum or by weekly deductions. It will not go to court; what you're reading is that all amounts over 5k are send for consideration for prosecution but, believe me, your case would never get that far!