You can also get a difference greater than 20C if the pump speed is too slow, which is a boiler problem on combi and system boilers.Return the boiler to factory settings. Set to max output (hold button in for 10 secs)
Check temperature of flow & returns simultaneously (no more than 20 degrees difference) if more than 20, it's a system problem, if less a boiler issue
The boiler does not stay on long enough for there to be a significant difference between flow and return. It might be OK when the boiler starts up first thing in the morning, but that's at 5 am and I'm too old to want to get up then.