Thanks for the reply, in answer to your question why the letter of authority was sent back to us, was because at the time he wrote and sent it to the tax office his current address (prison) did not match the address they had on the their records which was ours.
so the priority became to get my son's address changed to the prison address so he could deal with the tax himself, it was the miss match of address's that caused the delay in the first place when trying to fill the original tax returns in, they originally sent the forms to us, we told them he was in prison and informed them several times by phone and letter what his new address was. I would estimate this took 5-6 months. Therefore once this was sorted the letter of authority became less important as he could now communicate with them.
For the record he has now filled in and sent his self assessment with the help from the prison, however it was late and he missed the window to appeal against the charge of "being late" Hence the late the charge and daily accumulation of interest