The wider issues first:
1) People quote the Planning Portal as if it's the definitive work on PD. It isn't. It's a bloke at the CLG disseminating badly drafted Statutory Instruments, written in haste by civil servants, none of whom have ever submitted an application, nor had determine to one (I had a senior CLG officer work shadowing me and my colleagues earlier in the year 'to get planning experience'). The Planning Portal carries as much weight as any other website interpreting the General Permitted Development Order. It's a guide, nothing more.
2) If CLG had intended the 20cm to be measured from the edge of the roof slope, for heaven's sake why didn't they put that in the GPDO? The specific reference to eaves makes me interpret the Order as referring to the undercroft beneath the roof overhang. I know a lot of you don't agree with my interpretation, but it's not an unreasonable interpretation.
On your more specific issues:
Enforcement is referenced to when the building works were started not completed. The PD rights at the time are the ones are applied, not any subsequent revisions.
Whether the window introduces any privacy or overlooking issues is irrelevant in the Council's determination of your application. You simply don't have the PD rights to install such a window. Had all other aspects of the dormer been acceptable (in the Council's opinion) the Council would probably decide to take no further action or invite you to submit a planning application to retain the window.
It seems to me that your window is the least of your worries.
1) People quote the Planning Portal as if it's the definitive work on PD. It isn't. It's a bloke at the CLG disseminating badly drafted Statutory Instruments, written in haste by civil servants, none of whom have ever submitted an application, nor had determine to one (I had a senior CLG officer work shadowing me and my colleagues earlier in the year 'to get planning experience'). The Planning Portal carries as much weight as any other website interpreting the General Permitted Development Order. It's a guide, nothing more.
2) If CLG had intended the 20cm to be measured from the edge of the roof slope, for heaven's sake why didn't they put that in the GPDO? The specific reference to eaves makes me interpret the Order as referring to the undercroft beneath the roof overhang. I know a lot of you don't agree with my interpretation, but it's not an unreasonable interpretation.
On your more specific issues:
Enforcement is referenced to when the building works were started not completed. The PD rights at the time are the ones are applied, not any subsequent revisions.
Whether the window introduces any privacy or overlooking issues is irrelevant in the Council's determination of your application. You simply don't have the PD rights to install such a window. Had all other aspects of the dormer been acceptable (in the Council's opinion) the Council would probably decide to take no further action or invite you to submit a planning application to retain the window.
It seems to me that your window is the least of your worries.