Welcome to APUG Simon.

The overexposure might be due to:
- dirty or oxidised contacts on flash or camera;
- subject was too near | flash too powerful | film too fast | aperture too open, so to say, so that even the minimum flash duration gave you too much light. In that case, if your flash can do it, you will overcome the problem by using the flash at half power or at "winder" position.

Contacts can be cleaned with a pencil eraser (or pen eraser, which is rougher) or alcohol, applying friction with energy. Seize the occasion to clean the battery compartment contacts as well.