Further testing with old project files has shown that before i was in fact unknowingly using a screen percentage of around 175% (probably set when i started learning UE4 and didn’t know what was going on) and so when i went down to 100% in 4.18 ( because the console command hadn’t been executed yet) i suddenly noticed the low resolution.
It is strange however just how poor using r.ScreenPercentage = 100 actually is.
Answer is just to use r.ScreenPercentage = 225 if you can and lower if performance suffers