I haven't read the TP reviews, but it would be difficult to put negative "install" experiences directly down to E.On since the physical install subcontracted to someone local. Obviously E.On are involved in the comms and paperwork so they do have some responsibility. But important to look at the specifics of the complaints.
My 12 panel/8.2kWh install was £9k, but that was a year ago probably before interest really spiked.