Hi, sorry for the delay in responding.
Perhaps easiest to have a chat on something like zoom if you want the most details?

However, the short answer is by putting together information from:

* crop markings in aerial photos,
* pipe marker seen at field edges,
* photos uploaded to geograph.org.uk,
* public domain information from gas suppliers, such as their notifications to industrial customers as to when main gas routes will be out of commission for maintenance,
* walks through farmers fields.