HMRC Ignoring Complaint

HMRC is refusing, without giving a reason, to provide advance funding of SMP to a client. I made a written complaint in May 2017 but have had no response. I recently rang the number on the "Complain about HMRC" page and, after getting passed around a bit, I got through to someone who told me they had no record of my complaint. I understand the next stage is to complain to the adjudicator, but to do this I have to have exhausted the complaints procedure, which I have not done, as they have not allowed me to access it.

Can anyone suggest whom I should raise my complaint with next?

Thanks in advance for any advice.