A further update on this case:
I removed the covers at the weekend and can confirm the fans are working fine. I checked the large barrel / radial fan at the rear of the oven, and also the oven fan itself. I also checked over the connectors on the boards and reseated them where possible.
The problem still persists. When using it today the device operates for several minutes and then the oven light starts flickering, and you can also hear power to the fans stuttering as well. Eventually the whole device switches off including the control panel. It will come back to life after a couple of minutes. The problem does sound heat or dry joint related….