I had a account with Barclays it was one of the bundled accounts an one of the services kept charging me that I could not cancel so easily,
I incurred overdrawn fees, dd returned fees an interest that all snowballed until I went onto a DMP, later they got DSAR'd and a complaint sent to them.
Eventually they wrote me a lengthy reply to my complaint and phoned me several times to discuss it.
One of the issues I had was they attempted to contact me by but my details had changed but they did not email or try my home address.
Another of the issues was they closed a account down of mine with money in it (that I was not able to retrieve).
To be fair they were good in handling my complaint an I got it resolved an received compensation + interest up-to the same amount of all monies deemed owed & paid directly back to me.
The amount in question was below £2k but more than £500, it took me around 5 years to take action on this account so I am well pleased with the result of my efforts.
I may post more info on here later to help others