As an HMRC employee...
Can I first ask that you don't blame the general staff.
We have to work under the laws passed by the Government and in an organisation which is being destroyed by it's own masters and board.
The vast majority of staff really want to help you but are constrained by what is happening to the department. We remember the days when we were taught it was about making sure people "paid the right amount of tax at the right time" and that was the same with repayments, we also believed in "whole case working" whereby we didn't just deal with the query you had but we looked at your tax affairs as a whole and made sure everything was right.
If you want a better HMRC by all means complain about the legislation and the Government of the times actions, but even better why not support the HMRC staff and their union, the PCS in their current dispute with the HMRC Board and treasury. We are not only after better pay and conditions for ourselves but a better service for you. If we have the right amount of staff doing the right jobs with the right logistics then you would have your query answered correctly and competely in the shortest amount of time.
And if you think the HMRC is bad take a look at the other Government departments, they like us are being destroyed and they like the HMRC are your public services.
I'm sure you'll understand why I'm posting this anonymously, personally I don't want to be one of the approx 100,000 Civil Servants to lose their job.