It depends how big the storage heater is (regarding volume of water stored)
But rule of thumb, I'd say no, because even if it is currently within the guidelines, whose to say one day it may upgraded to a higher volumed vessel and the additional socket outlets are not then removed it then becomes a problem.
So in that regard Part P is no concern, as no work is going to be carried out.
If your in need of further sockets, I'd be taking them from another suitable circuit if possible
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