I have a 1997 cherokee limited 4.2 petrol. When I first turn on the ignition all instruments come to life then die but can be brought to life again except the speedo and rev counter by pressing the 'lock' button on the remote key lock.
Is this an earth wire problem?
Where do I start looking and why should pressing 'lock' on remote have this affect?

 

I would first pull the dash apart and remove the instrument cluster. It has pins on the back of it that push into a connector in the dash. Clean the pins on the back of cluster and the receiving connector in the dash with a proper electronics contact cleaner. Also, check all appropriate fuses.

There are some videos on YouTube to show you how to disassemble. I would try that simple fix first.