the earth is in one of the pins to the clocks, check my fusebox FAQ in the garage section. The braided earth from the rocker cover to the coil bracket also affects the dash as well, so check that too.
The battery may be fully charged, but that doesn't mean it isn't broken and not holding enough to keep the clock alive when you crank the engine

I remember our Polo had this problem as well, turned out the alarm was draining the battery...