Thanks for the help and advice so far

I did manage to resolve this by -

1). Installing the PINN multi-boot software from the Raspberry Pi Imager

2). Using PINN software to set-up a dual boot set-up on the SD card

3). Installing -

3.1. Raspberry Pi OS
3.2. Retropie OS

4). Adding the various games and ROMS that I wanted on each OS / partition.

@mitu oh ho, I know it's not supported. I tried it as my main configuration for a while and eventually it just would not start any games at all. I'm hoping that because it's on a secondary sd card it'll fix the issues i previously had.

If i had a choice, I would totally prefer to have it on top of raspbian but, whatever.

@rbezares I had this a couple of days back, somehow es had managed to remove it self, I refreshed the card and it happened again, although got slightly further in the boot as es did appear before crashing. I flashed a new image to a different micros and it has worked fine since so am putting it down to a bad card

The default cmdline.txt depends on your RetroPie and Raspbian version, what version of RetroPie are you using and which image did you use to install it ?

My bad... I am sorry.