Photograph the lawn as a whole and take closer views of the affected patches. Describe when you first noticed the problem and whether it returns in the same places. Note areas used as walking routes, shaded sections and spots that stay wet after the surrounding ground has dried.
If you know about earlier treatments or ground work, include that information. There is no need to diagnose the problem yourself. In a London garden, buildings, trees and narrow layouts can create very different conditions across a small lawn. Show the entrances too, particularly where equipment would need to pass through the house or down steps.

