Rcd trips after 1 minute of using top oven or grill

The grill element has an internal short and needs replacing. Sometimes you can see it as a slight bulging of the element. This happens just before it blows a hole. It may also have been physically damaged when something was put in there after a few Sherries. :>D Sounds like your RCD is adequately sensitive.

An RCD tripping points to an earth leak because that is what they are designed to do.
Check the wiring and pay special attention to the insulation.

Rcd? do you mean the circuit breaker? if so the grill element is shorted.

The only thing left sounds like the thermostat. Several online appliance parts stores are very helpful.

Check fan oven element

The elements in ovens consist of an outer earthed metal tube and an inner resistive element that is insulated from the outer tube by what is effectively a compressed ceramic insulating powder. If the element is not used for a while, enough atmospheric moisture, (or from spills, etc), can penetrate the insulation to cause enough earth leakage to trip the RCD. If the RCD is tripping at the correct level, (They can get too sensitive - see this article here:
Diagnosing RCD earth leakage breaker nuisance trips), then the only long term solution is to replace the element, although they can sometimes be dried out by heating them.

just the oven? then more than likely the element. if the stove eyes are not working as well, then your looking more at a sensor, thermostat, fuse, breaker, etc. almost always an element though

One blogger found that the insulation on one of the fans was breaking down when the oven warmed up. You are only going to find out what is actually happening with the oven out of the kitchen carcas and using the services of an electrican to test and see what is happening.

We need wiring diagram for HB905 Siemens double oven, to start with it kept tripping, element ok. Now we have no heat coming from the main oven.

Hi, there's definitely a electrical problem.
First, use the bottom oven only for about 15 minutes and check if it doesn't trip.
Second, use just the grill (top oven) for about 15 minutes and check if it doesn't trip.
The reason is sometimes grill elements are corroded and only trip after a few minutes.
Also, if both ovens work ok on their own, then it could be your fuse box not strong enough, or the isolator switch fuse be only 13amp. If your oven is more than 3Kw (check the label on the door for info) then you need a 30amp fuse to hold when both ovens are on.
Cheers