pump is on return and a 2 port electrical valve on flow ... this is there as an ant-thermal syphon valve to prevent hot water flowing by thermal loop back to bolier when boiler not on burn.
A check valve directly after the pump can do the same and cheaper and less problematic.
However, is the pump OK?
Is the zone valve OK?
Is that sticking?
Is the pump switched via the zone valve end switch?
A pump can stick when started after being off. If it sticks, then the burner is on and an overheat situation. A sticking pump is difficult to diagnose and intermittent.
As your system is clean (no ferrous except the pump, I think), I doubt you will have bearing problems. but you never know. Inhibitor makers claim their products lubricate pump bearings. Is there enough inhibitor in the system?
Grundfos have ceramic bearings, while Wilo are carbon (plastic). Dirt in the system will degrade Wilo pumps quicker than Grundfos.