Hi Nex
Different people handle complaints, and they will have no influence on your MR or a future appeal.
I'm sorry to hear that you had such a bad time. Only you can decide whether it's worth it or not. Do you know what outcome you want? Most complaints don't lean to the claimant getting what they want, but that doesn't mean you shouldn't try. The DWP rarely acknowledge mistakes but occasionally people get a mealy-mouthed response saying that a report - didn't meet their normally high standards!
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
