Just realised I completely forgot to post the solution.
Long story short, decided to revisit what
@nzenigma mentioned regarding fuel system so took it to another garage owned by a mate with better equipment and better knowledge than the other guy. He had his "technician" have a look (previously away on vacation) with something fancy.
The "nearly new" injector that I noted was in there originally, turned out to be a badly remanufactured injector and it wasn't working properly (to be blunt). It was working enough to make the car "work", but very very slightly off which buggered starting in a specific temperature range and caused the misfires, so I took it back to the indie dealer and told them to switch it with a used, low mileage OEM injector from another i30 they'd just acquired with severe bodywork damage (assuming they were going to use it for breaking, by chance). Same model, year and engine type, lower miles but in red
Dodgy injector that wasn't dodgy enough to make itself known to whatever crap the indie dealer mechanic was using for diagnostics (very tiny discrepancy).
Problem solved
