An error has been identified in the written answer given on 29 January 2019.
The correct answer should have been:
The cost to taxpayers of delivering the 2016 EU Referendum was £129.1 million. A breakdown of that cost is provided in the report published by the Electoral Commission.
The then Government undertook publicity on EU membership ahead of the referendum. This cost an additional £9.3 million.
The total cost to taxpayers was thus £138.4 million