Here’s a leafmine I hope you don’t find but you probably will, and easily. So, why do I hope you don’t? Because this mine is found on plants of the Impatiens species, including the highly invasive plant Himalayan balsam (or Indian balsam, as it’s officially known; i.e. Impatiens grandulifera).
The leafminer is the larva of Phytoliriomyza melampyga, which creates an initial narrow twisting gallery that then widens into a variable whitish blotch. The larva’s frass can easily be seen within the mine. The adult fly is actually rather cute, with mostly yellow, a bit of black colouring. (You can see a short video of it on Wikipedia.)
The NBN Atlas is currently showing just 192 records for this species in Britain, though, like most flies, it’s probably under-recorded.