As Jarvis put it, China "has a low threshold for information it regards as valuable". This is because over time it builds up a wider picture by piecing together the morsels it may extract from a wide range of people.
Labour, since they won the general election, have attempted to warm up the UK's relationship with China.
The Chancellor, Rachel Reeves, has been to Beijing, as has the Business Secretary, Peter Kyle, who was there in September within days of being appointed to the role. The most senior civil servant in the Foreign Office, Sir Olly Robbins, was there last month in the midst of all those headlines about the collapsed spying trial.
But there has long been a vociferous collection of China hawks in Parliament, who have long worried about what they see as a collective naivety about Beijing.
The question now is whether a moment like this, following MI5's intervention, persuades others to share their outlook.
The government insists its approach to China is "pragmatic": it regards working with Beijing as inevitable but insists it is "clear eyed" about the risks.
Will a growing number of MPs demand a more sceptical outlook?
As one put it, rather colourfully, "you can't reason with a tiger when your head is in its mouth".
China is a vast superpower that is at once essential yet awkward, unavoidable yet dangerous.
This the latest case study in those competing interests. There will be more to come.
