The oven and grill when in use switch themselves off after a while. They will switch themselves back on after a while.
It could be the thermal cutouts operating. First things first, is the cooling fan running and does it turn off about 10 mins after the ovens/grill are turned off? If it isn't running then the unit is overheating and the thermal cutouts will operate. If it is running normally, then the cutout could be switching in early due to them becoming old and ineffective. There are two cutouts in the oven. One controls the main oven and the other controls the top oven. However, the grill operates without the clock being set, i.e. it is independent of the clock. So, if both the top oven and grill stop working together, according to the wiring diagrams in the handbook, the only thing it could be is the thermal cutout for the top unit.
F.Y.I. The top oven switch controls both the grill and the oven elements. In other words, in its OFF position, the Grill works and the Oven doesn't and in its ON setting the Oven works and the Grill doesn't. However, I can't see this failing as you specify, but worth checking if you take the oven to bits. The Grill will only work if the oven door is down.
If you need to replace the cooling fan, you will have to take the whole unit out of the kitchen cupboard to gain access to the top and rear oven panels, which have to be removed to gain access to the cooling fan and wiring. If you do replace it, ensure all the vents for air circulation are free from dirt and grease before you put it all back.
Try espares for STOVES spares. If the item is not listed you can ask them for a quote.
Be aware that the cooling fan is controlled by a timer circuit board and an in-line resistor on the fan terminal block. Either or both of these two and or the fan could have failed if the fan is not running! Any competent electrician should be able to work out what has failed.
All the diagrams are in the Handbook. If you need a copy, supply an email address and I will send you a pdf copy.
If you do ask for a quote, I suggest you ask for prices for all the parts mentioned above, so once you have checked what has failed, you will have enough info on pricing and availability as to whether to proceed or not.