Stoves 800DIS Cooker?

We have one of these cookers in our Holiday Home. The oven will not heat up properly (about an hour for a limp warm pizza).
Burner and thermocoupling and top thermostat appear undamaged.
Two 1 inch holes either side at the top of the oven; vents, obviously; but should they be wide open and unshielded for all the heat to escape? I don't see how the oven can heat up with all the hot air escaping. Is there something missing?
Anyone able to suggest a cause and cure, please?