The diverter valve needs to be checked for correct operation before any flushing is done!!
Sounds like it is sticking or fooked.
Is it the VC series head (plastic) or V404F type.
It breaks down every couple of years ........Does it get a service in between??
300 quid a year for a breakdown is a lot for an oil burner.
What Boiler /Burner combination is it.