Can somebody tell me which light is between the 'Parking Break' & 'Oil' light on the dash? The cover with the wording is broken and that light keeps coming on. It is a 1974 XJ12L Series 2

 

This is the brake Warning light. It's hooked up to the pressure differential block on the frame rail below the master cylinder and to the fluid level sensor. It normally comes on when the shuttle valve offsets because of a difference in pressure between the from and rear fluid brake circuits or if fluid is low. If it's on all the time check the fluid and fluid level sensor.

 

You might bleed the brakes front and rear to equalize the pressure to turn the light out. It's letting you know the bias is wrong and frt or rr needs to be bled to equalize and reset sensor.