Remember though that the oven and hob draw a LOT of current.

They should be on their own circuit with bigger wires and a 30amp 'fuse'.

If the RCD is not big enough, could this be why it is tripping?

Thanks.

The Hob and Oven are on the kitchen 25A fuse/breaker. This does not trip.

Only the main RCD trips when the oven is on (presumably because of the earth leakage). The RCD says 30mA.

I guess I will note the resistance reading from L/N to earth, and do a bit of Ohms law to see if correlates to more than 30mA.

The RCD which by design is very sensitive and is there to prevent someone being electrocuted, it trips when there is a difference in current between the live and neutral caused by current going somewhere else.
The main fuse 25A is to prevent overloads

You could get a 100 mA RCD ,but that doesn't seem the issue here.
If the oven is starting to have earth leakage problems ,it could be a good time for a new one , by the time you locate the fault(s) source replacement parts if that's still practical or pay for an engineer you may not be much out of pocket.
Ask your better half what they would prefer ,I think the answer will be NEW please

Restaurant every day

Strictly speaking the fuse is there to protect the cable and prevent overheating and fire.
[Sorry, I can't help being a pedant ]

Cable overheating is due to it being overloaded?

Don't ovens plug in to standard wall sockets?

Only if you want to burn your house down

Although, to be fair, there are a few very small ovens and combi-microwave ovens which would plug into a wall socket but all serious ovens need a dedicated circuit - usually 30A.

I had a similar problem with a Corbero oven, when it was set to auto it tripped the RCD. By setting it to manual it functioned fine. If your oven has an LCD panel have a look at the settings and refer to the manual. Failing that find a family member's birthday and come up with a "surprise" new oven.

Everyone I bought in Spain came fitted with a standard plug

Ovens do not usually come with plugs on.

I have just bought a new oven and hob and neither had a plug on.

The previous ones did not have a plug either and they both are connected to a dedicated 30a circuit.

The only dedicated circuit I have come across was one I wired myself all the others have had a normal 16 amp socket fitted below the workbench behind the oven.
However all of these had gas hobs until as mentioned I installed my own circuit.

Never thought of that Fred, hope my wife doesn't read thread , she has been asking about a new cooker recently

Anything that draws that sort of current - oven, hob, even air conditioning units should be on their own 30A circuit.

The plug and plug sockets for ovens and hobs are VERY different to the usual ones (they have an extra 'prong').