Rishi Sunak was asked at PMQs about axed 2016 investigation which might have cleared postmasters sooner.
Henry Staunton has released a note of a call with a senior official, amid a row over Horizon payouts.
The new notes have been printed ready to use and will be used for the first time on 5 June.
The popular remedy risks "small particles" entering the phone and causing damage, new advice says.
The £16.7bn surplus was double last January's but lower than economists expected.