Any electronics boffins?

[Andy Clap]

Diode may be reverse-polarity protection - but it may also be simply acting as a fuse. Its not something I'd replace with wire however.

It may be burned, but it might still be semi-functional - it can happen.

If you have a magnifying glass, you may be able to make out any identifying characters (eg 1N XXXX).

Final point: although you've identified the burned out diode, it'd be a good idea to make sure you've identified any underlying faults to prevent the new one going the same way.

Fuse - well that would explain the F1 annotation.
It was smoking with power applied and burned to a crisp, not funtioning at all I wouldn't think.  Fell apart as soon as I clipped the legs, only readable bit was "6KE".