So I finally discovered that the issue I was having with my N64 was caused by my Otaku Games Switch.
It’s no longer accepting the RGB sync on luma signal, and just outputs a buzzing blue image but I borrowed a SCART to BNC break out and hooked it up directly to my 20L2 and it worked first time with no issues. The interesting thing is that all my other consoles work, I’ve mixed and matched inputs, tried a new cable, a new power supply.
I’ve started the return process with Otaku games so it will be interesting to see how this pans out. I’ll be sure to post my experiences here.