I have just soldered the backpack onto this LCD Negative red screen.....
I can get the Hello World code to run on the Arduino and on the screen however the LCD backlight is NOT working..... Can anyone suggest anything I can try?
I have tried various libraries and permutations of code today but not even so much as a flicker from the backlight....
Is there a way I can "bypass" the backpack and connect the screen up to try and see if the screen backlight is actually working?
You have not provided any details on the h/w you have nor shown any photos.
It sounds like you have a i2c backpack.
There are several possible issues, misconfigured library, bad soldering, bad backlight led, burned up backlight.
It is possible that the LCD you have does not have a current limiting resistor on it which could cause it to be burned up by sending it too much current.
Not all backpacks control the backlight the same way so it could be that the library you are using is not configured for the backpack you have.
If you have a i2c backpack, you could try my hd44780 library.
It can be installed using the IDE library manager.
You can read more about here: GitHub - duinoWitchery/hd44780: Extensible hd44780 LCD library
The i/o class is hd44780_I2Cexp
It should autodetect the backpack and determine the correct backlight active level to be able to light the backlight. (other libraries do not do this)
The hd44780_I2Cexp i/o class includes a diagnostic sketch (I2CexpDiag) which will test the i2c interface pins, and internal memory of the LCD.
It will also blink the backlight to test that.
If you don't see the backlight blink then the backlight may have an issue.
It could be bad soldering, or burned up backlight from the LCD module not having on board current limiting resistor.
or even something else.