Hi,
If you have a mains transformer in a product for 350W of real power, (transformer/rectifier/capacitor power supply) and its approx 120mm cubed..is it likely to be useful for 50Hz mains and 400Hz mains?....i thought a 50Hz transformer would tend to overheat if put on 400Hz?

 

Generally speaking (writing), 50Hz transformers and 400 Hz transformers are completely different animals. Running a 50 Hz transformer at 400 Hz is likely to result in excessive eddy current losses in both the wires and the laminations. You can try it but make sure to have a fuse in series with the primary.