So smooth…. great instructions iFixit crew!! In a past life I was strictly PC and built/modded a loads of systems over the years, but never a Mac. To ease the tech-xiety I copied the old drive to the SSD in a USB case, and then booted from USB for a couple of weeks until I was sure there were no surprises. Swapped the SSD for the HDD and it was like the heavens opened and the choirs sang (Monty Python choirs of course). And thanks everyone posting, for your tips and lessons, they definitely came in handy.
I’m now looking at swapping the optical drive with another SSD, and then maxing out the RAM. Where will this madness end…..???? :)
K