I had a very similar case, where the clmt reported the change to the DWP, I used the same arguments that you are looking to include and [color=red:5cb54f7cf7][b:5cb54f7cf7][i:5cb54f7cf7]lost[/i:5cb54f7cf7][/b:5cb54f7cf7][/color:5cb54f7cf7] the case.
The reason being that the clmt stated that the DWP member of staff he spoke to stated that they would inform the LA of the change, so because they didn’t, TAS said that was an ‘offical error’ by the DWP member of staff , and as they made the mistake and promised to advise the LA – the overpayment had to be made unrecoverable.
I tried to argue the LA case using the ‘need to report to the authority responsible for HB’ etc – but was still told to make the opay unrecoverable from the clmt.
TAS stated the principle of Sier didn’t apply as the staff member had promised the clmt that she would definately advise the LA of the change to the household
So, good luck 😕