I am tryiing to replace the power supply in an advebt mini desktop no fp9004. I have the case open but there is a metal plate across the ibside of the case which I cannot remove

Denis Jones, October 2011

Maybe you removed the cover on the wrong side.

Open the cover on the left side. The power supply should reside in the bottom left corner.
If it laying flat, with the front toward you, the power supply is on the left toward the rear.

Here a link showing how it should look like:
http://www.pcpro.co.uk/gallery/reviews/354373/advent-firefly-fp9004/141499

Then, it's only a mather disconnecting the power connectors, removing 4 screws (maybe 5 or 6 in some rare cases), swapping the power supply, puting back the screws. and reconnecting the various power connectors.

Electro, October 2011