Many thanks for taking the trouble to explain. I appreciate that!
Ok, I can now say safely that I'd evacuation done before removing the old compressor, and it was charged with R12 after assembly. I've been driving the car for about 1 month ever since.
I was at the workshop all the while during the repair, and at no point in time did I see my mech carrying out the "vacumm leak test" as mentioned by Steve and Larry. Perhaps they are not even competent with this particular procedure!
Will it be alright to go without the "vacumm leak test"?