Whilst most of the roads in that estate appear to be adopted highway, there appear to be parking areas just off those roads. As a matter of interest, is the location you turned into this?
If, so, even though those images are over 18 months old, it would appear that this parking location is not part of the adopted highway but rather a privately controlled car park. Some of the signs on the boarding would appear to possible be terms signs although it would not stand up in court as there is no entrance sign warning that this is now private land and to seek terms and conditions signs.
Additionally, what does your sisters lease/AST say about parking, especially anything about parking for visitors. What it does not say about parking is equally important. It is extremely rare that CPM has any overriding tight over your sisters lease and right to quiet enjoyment, which includes any visitors.
I suggest you or get your sister to take some photos of the signs as you run into the location where the ANPR camera pictured your vehicle and then any other signs within the parking area that have terms and conditions on them. The signs will (must have) the name of the operator (UKCPM) on them.
Ignore the streets with the official signs. They are adopted highway and not relevant to anything to do with private land/car park.
If you follow the advice you receive here you will not be paying a penny to UKCPM, but we need to see the actual entrance (if any) sign, terms and conditions signs in the parking area and precisely what your sisters lease/AST says about parking.
