jopp

I have tried registering my children for a Self Assessment online, but it will not allow me as the applicant is under 15 years and 9 months. I have since got their UTR number and NI number. I have then sent off a paper tax return as I was unable to do online. This has just been returned, saying that "while it is ok for a mother to sign, we do not have her as a third party on the record." There is no information what I need to do on the letter about how I add myself as a third party - could someone advise please? They have given me a deadline of returning the completed tax return by 8 May. I have been on the self-assessment line now for 45minutes.