@sungod
No, it is not an unlimited data sim.
Yes, I have been able to top it up for the last ten months. And, as a result, the validity keeps getting pushed out to a year in the future.
I was just concerned that after a year of use, the sim/number would stop working. I am now realising that is not likely to happen and I was misinformed or I misunderstood. Either way, it is great as I have no wish to change my number and all the hassle that that entails.