I can understand that Network Rail can prevent trains running during the night and that opening train stations on the route can quickly get very expensive.
Chartering buses / coaches is not expensive and a way to test demand before making a bigger organisational change - also keeps Network Rail out of the frame. What motivations would Greater Anglia / Abellio have towards operating coaches, stopping at maybe 2 stops en route and which would accept train tickets as valid ? Would the Dept of Transport or a rail regulator have any cause to object either ?