CSA ‘estimated’ my earnings and now say I owe thousands

I am having a lot of problems with the csa and apparrant arrears that a based on estimated earnings that they believe I in the space of a year as a self employed joiner in the resion I was earning over £1200 per week and I owe £6500 in arrears. My actual earnings were closer to around £200 per weekI have sent all of my prove recorded delivery witch they are saying they never received even tho letters were sent out confirming  receipt. Now they are saying they can’t make any adjustments because it has gone over 28 days can any one help I don’t know what else to do.