Wrong name on old children's savings book - help

I opened a children's savings account in 2005 through the post office with NS&I. When I received the little grey book it had the wrong initial on it, it should have read B T Evans but instead read H T Evans. I mentioned this in the post office and they said they would amend it. Fast forward to 2017 and I remembered the account. I went to the post office to take the money out for my son and was told the accounts are now closed so I had to contact NS&I to close the account. Because my son is now over 18 it has to go into his bank account which is in his correct name and doesn't match the children's account (the post office obviously never filled in the firm to change his name !). They have said they will not pay out the money that is in there. We have sent every form they have requested, sent proof of my signature, address etc and they still refuse to pass over the money. Any ideas of what I can do? I still have the grey book but that is all I have.