+1 on a human at HMRC can be helpful.
That said, I have experienced amazing incompetence and what I considered deliberate nastiness from HMRC, leading to both investigation of 4 years of my self employment (I was 33p out over 4 years, in my favour, all documented) and all sorts of weird shenanigans around tax credits as well. The final Tax Credit conversation with a manager made me realise that there a different levels of both competence and authority within HMRC, with the manager finally admitting that no-one really understood the calculations or why a computer said we owed money still, when in fact it was likely I have overpaid back again and seeing as it was 5 years in the past we should all move on….