Earlier quoted context omitted.
They probably couldn't even tell him how they caught him, because then they'd have to admit that GCHQ is spying on most government members, too.
Given the technical aptitude of most politicians in the UK, he may equally likely have used a government-issued device
https://www.bbc.co.uk/news/uk-politics-15396956
Letwin had a habit of reading correspondence whilst in a public park in the mornings and then dumping it in public bins once dealt with. Mirror reporters just followed him around and fished sensitive material out of the bins.