On a 57 plate suzuki swift the speedo and mileometre have stopped working.the fuel gauge is up and down.and now the diagnostic light has come on.any ideas

This is usa forum and car is UK
our cars are diffr here, in many ways.
not withstanding that fact, ill answer, for USA cars.
fuse 21 loose>? the ignition switch power feeds 21 and 21 feeds power to cluster, gauges. tacho, temp fuel and VSS sensor.
DTCs
P0500 Vehicle Speed Sensor Malfunction
P0501 Vehicle Speed Sensor Range/Performance
P0502 Vehicle Speed Sensor Circuit Low Input
P0503 Vehicle Speed Sensor Intermittent/Erratic/High
(usa OBD2 codes yours may be different but words same)

stopped working does this mean the speedo needle is stuck at 0 kmh?
if it does not move at all moving, the cable is snapped.
or gear on far end of cable is bad or its key stripped as many do.
we check that the cable inside turns, as car moves (on stands)
if it moves and cluster at 0, the cluster is bad.
the transaxle is the source of the cable drive.

i think it has 2 problems.
bad fuse21 or key switch
and bad cable.
fuse 21 runs the fuel gauge. all gauges, but speed.

Check for a loose cluster fuse first.
its losing power.
if you scan it ,it would say?
check fuses first. the scan it.

Its the speed sensor on the gear box had the same problem last week it cost £147 at a non Suzuki garage problem fixed.

This sounds like the instrument cluster has gone faulty, unfortunately. With any luck, you might be able to find a replacement at a scrap yard somewhere to at least save some money. Otherwise, the dealership is the only other place in which to purchase one of these.

I hope this helps and good luck! :)