Speedometer, Rev counter, Fuel & Temp gauge, horn do not work

[MartinObviously]

I think your horn is probably unrelated to your dash fault, check the plug on it for corrosion.

If you've got a newer saxo with the digital dash (it has an LCD screen on the speedo face) then it's likely the dash has simply failed and needs replaced. Not that uncommon.

With the ignition on, pull the bottom right most fuse in the fuse box and reinsert it, and same for the bottom left most. that might get the dash working again.