I have, with varying degrees of success.
I have taken on the DWP over an alleged 'overpayment' of £5225 which they proceeded to deduct from my benefit.It took me 4 years and several Freedom of Information requests to eventually prove that not only did I not owe this money but that there were serious, if not criminal, actions but certain DWP employees. As is the case with all bureaucracies no admission of guilt or fault was forthcoming.
I have an ongoing complaint with my local health authority after treatment left me permanently impaired. Again it was necessary to use the Freedom of Information act to get to the truth because, as has been very evident recently, health board bureaucracy will use any and all methods against anyone making a complaint. I have been accused of everything from lying to being mentally unwell but their main weapon is time. Although they must answer the initial complaint within 20 working days or let you know the reason for any delay. The chances of a complaint being resolved in this 20 day period is extremely unlikely and there are no guidelines n how they respond to follow up correspondence, which means that they will take months to answer even the most simple question.
I also learnt very quickly is you can only send letters via recorded delivery and never discuss anything via the phone. In regards to letters it is amazing how many they do not receive unless they have had to sign for them and in the case of telephone communication a denial of what was said is common practice.