Morning Clemens, on the Plus Six there are what looks like 4 primary fuses located in a black plastic box thats positioned on top of the battery, the Positive ( red) feed cable from the battery goes into this primary fuse area, power is then distributed out through primary cables, these power cables then feed other areas of the car, which I believe also includes the main fuse box. If the Plus Four is the same design and I think it is, then it might be worth checking these with a test meter if you are sure the battery is ok. If you find that one of the fuses has failed, that may not be the only issue, as it would take quite a power draw to blow these fuses. Also check the earth lead from the battery to the alloy panel is tight. The Plus Six and Four have had a few earth issues.
Sadly Morgan have not (yet) shared wiring schematics for the Plus Four or Plus Six to owners, so its either a case of trying to trace the power ( or lack of ) with a meter yourself or having the main dealer look at it.
