From the shape, flat but bulbous, it is much more likely to be a Bramley than a Grenadier which is a much more rounded shape. Grenadiers have largely green skin without the red tinges a mature Bramley possesses. The large tree size, vigorous growth, large apple size and heavy cropping indicates a Bramley but badly-grown trees could lack these distinctives. Make sure you give the tree sufficient room and remove crossing and any diseased or dead branches, otherwise a mature tree needs little other care.