Hi Its MamaTax Here!
Thanks for the question.
However its difficult to advice not knowing how the underpayment arose; whether there is indeed an underpayment etc. My advice is to get a tax investigation specialist who will provide you with specialist advice to minimise the stress and anxiety that HMRC may be causing by going to and fro about the past underpayment. You can easly get someone for say £100- £200 on sites like this LINK where you post your requirements for no fee and the speciliast appriach you - visit /call and go through with you the past accounts and rework tax paid and what should have been paid. They will advice whether to appeal or not . In fact a specialist takes over all the communication with HMRC and i guess this a more efficient and cost effective manner of dealing with your case.
Hope this helps. If you have any further comments please let me know.