Hi Jacek,
I’m getting a little further with my own query which is currently with GWR as they set the fares and cap extension fares for all the branches off the Reading mainline. The problem is that no-one seems to want to publish what the cap extension fares are.
I am intending to take the matter up with either London Travelwatch or Transport Focus in the new year because the unpublished nature of the extension fares means it’s impossible to check that you have been charged correctly.
Keep an eye out on the site.