I was told by the CH Admin that my Mum was allowed £24.40 personal living allowance!!! So isn't this applicable with mum who will be paying FULL cost for the next 8-9 months.
Incorrect.
First, it is not the business of the care home administration to decide such matters. The only concern they should have is that your mum pays her bill. There is no question of their limiting whatever rmoney she may have left over from her income - if she pays the bill and has money left over then it is hers.
Second. so long as your mum is fully self-funding then whatever money she has is hers to use as she pleases. However, some care
does need to be taken so that gifts etc are not classified as excessive because in that case they may be regarded by the local authority, when they do the financial assessment, as trying to give away money so that it can;t be used to pay for care fees (this is called a Deprivation of Asset). The assessment would be done whenever your mum looks like she may qualify for local authority funding (partial or full) for her care home fees.
However, simply continuing a long-established custom of gift-giving to granschildren for special occasions should not be a problem, so long as the amounts are reasonable and do not (say) deplete her savings or cause a shortfall in her income, and are in line with amounts she has given previously.
Should your mum in the future start to receive local authority funding, then it is indeed correct that whatever income she has must be used to go towards paying whatever portion of the fees is possible - but in that case, a given amount each week is reserved for her exclusive use no matter what, in essence is is a form of "pocket money" that she may use as she wishes.
Although the amount is not huge, it should be enough to provide for birthsay and Christmas gifts, perhaps not as generous as they might have been in the past though.