As was suggested earlier, this suggestion by that well-known source of dubious financial advice, the hairdresser, is likely to be to do with payment for care in later life.
There is no substitute for proper paid-for financial advice, but unless this flat is worth substantially more than £300k, it seems unlikely your mother's estate on her eventual passing would be paying IHT.
You should be made aware though that she has to live or 7 years after the date of the gift to you for the full impact of IHT liability to all to zero. You said it happened around six years ago, and she sounds pretty healthy at present.
The likely issue, then, is care payments, as if someone gives away all or most of their assets, then expects the local authority to fund the cost of care, most authorities don't take that lying down, and they make strenuous efforts to track the gifted assets. With a house they would put a secondary charge on it such that mix much later when you sold it, they would be alerted via the Land Registry, then make their claim against incurred costs of care.
No hers will provide you further insight from their own experience.