Leaving the issue of posting aside, and concentrating upon other procedural matters, you say in your post of 18 November that "I have now seen a copy of the letter of 11 Jan, which does look like an initial enquiry letter."
Does the letter clearly state that it is a "Notice of Enquiry" issued following the provisions of TMA 1970 Section 9A and does the issue follow the HMRC guidance at
https://www.gov.uk/hmrc-internal-manual ... ual/em1551
or is it a "Request for Information" to check your tax position following the provisions of Schedule 36 FA 2008?
https://www.gov.uk/hmrc-internal-manual ... ook/chapp1
As the guidance at CH21680 notes "Information and inspection powers can be used at any time provided that the information or inspection is reasonably required for the purpose of checking a tax position", but the information an documents that may be requested under these powers is limited to what is reasonably required to check the accuracy of a person's tax position.
You will see for example from the guidance at EM1551 that where an agent is acting the taxpayer should be served with a formal notice of enquiry and the factsheet but the detailed enquiries will normally be sent to the agent.
The time limit for an enquiry into a 2014/15 return would expire on 31 January 2017 if the return was filed on 31 January 2016 but would expire 12 months after the day the return was it was delivered to HMRC if filed before 31 January 2016 - see S9A(2) and HMRC Guidance at https://www.gov.uk/hmrc-internal-manual ... ual/em1506
If the return was filed before 11 January 2016 the notice (if made following Section 9A) would be out of time invalid in any event.
In any event it is unlikely that HMRC are taking an interest unless they hold information that already suggests that there is something wrong with what was disclosed on your 2014/15 return. If you know that there was something wrong with the information returned then you have a duty to disclose it, irrespective of whether HMRC have followed their own enquiry procedures correctly. Even if you believe that nothing is wrong with your return, and accepting that HMRC should only ever operate within the powers granted to them by the statutes, If (and only if) HMRC have been clear about the risk of a loss of tax that they are concerned about the pragmatic and most cost effective solution may be to address the issues that HMRC have raised rather than trying to avoid them.