Thank you both.
MichaIng, my understanding was wrong as I was following this (now) outdated info, which I found whilst Googling:
That was why I mistakenly thought the DietPi optimised software install method would be stuck on 3.3.1.0 for now. I’ll keep an eye out for 3.5.3 if your testing goes well. Cheers.
johnvick, I unfortunately nuked everything and started again (short of time, wife wanting to watch something rather than me tinkering…). So can’t test this or pull any error messages. Apologies. The errors may actually have helped get some sort of answer too.