Hello Ruth, you may be better to harvest now if the hearts are solid. But it does depend on variety; for example I just harvested lovely heads of Marner Early Red, very dense. But there is also "Marner Red" which matures later and can carry on growing.
Whereas an early variety, well hearted, is less likely to grow any more and more likely to be eaten, not by you! (caterpillars, slugs)