HMRC complaints

I am writing a piece for the Daily Telegraph on how to claim compensation for terrible HMRC customer service.

It comes off the back of rising complaints about sub par service from the taxman but low levels of compensation.

I am looking for comments how how much compensation people could be entitled to if they have a complaint about HMRC andwhat steps you should take to secure a compensation payment 

Are there certain things you should say to make compensation more likely, and which kind of complaints are more likely to get a compensation payment. 

Many thanks

Marc