something has changed on my PC, maybe an update?...my colours now seem to be printing different...in Tools - Colour Management - Default Settings, does anything below stand out as to why the black isn't printing a pure black?
No, check the printer.
Thank you...another silly question, would it make a huge difference if the design was in cmyk?
With your Color Management setting all the RGB blacks R0G0B0 should print as a solid K.