Later, he followed Milne and the woman as they walked towards the park, hanging back to give them some privacy.When he went into the park, he said, he saw they were on the ground having sex.
Carlisle Crown Court was told today that his young victim was left distressed, upset and put off her studies.
Johnstone was sentenced today to 12-months in prison, having previously admitted causing or inciting a child aged under 16 to engage in sexual activity in December last year.
Parking is pay and display and there are over 1,500 spaces.
An online "vigilante" helped snare a man intent on committing a child sex crime.
The degree of culpability and harm must be at a lower level.