Sorry Harry, but if for any reason they have paid you anything over what you should receive, well you have to give it back.
The only thing that may be in your favour is they did not give you the extra £40 that your mentioned.
If it's wrong or right they are the best people to sort it out.
Many years ago I was awarded child credits I did not asked for them and I did not want them either, but they gave them to me.
Later they realised it was a mistake, and yes they wanted the whole amount back straight away.