Alright quick follow up for anyone interested.
The mechanic did a vacuum pressure test for leaks but couldn't find any. They said the gas was quite low, enough to get the compressor going but not enough to make things cold. So they regassed it and put some dye in to assist in future diagnoses of any leaks.
They also said the compressor itself sounded like it was on its way out so they gave it a little oil.
Anyway with just the regas we're ice cold!!
So I'll look at replacing the compressor at some point before it completely fails but I'm otherwise very happy at the outcome and have cold air in time for the Australian summer