As long as it is not weeping sap, then it just the bark. We have this problem with one of our old trees and it has had it for nearly 20 years without any visible diminution of fruit production. Just make sure that the split is clean. They are a great hiding place for codlin moth larva.
There are compounds available which you could us to fill the cracks, but none of the experts recommend them.

Thanks OB, i will look into suitable fillers.

I wouldn't fill in the cracks it will just encourage fungal growth and insect attack. Cracks are not uncommon and are usually caused by differentials in wood expansion and contraction due to changes in the seasons temperatures. The tree is capable of dealing with this on its own.

yes our apple tree is in a similar bark state and it has had no ill effect on the tree's apple production. personally I'd leave it alone. we find ladybirds and other useful insects lurking deep in the cracks overwintering.

I did say that filling is not recommended.

Thanks guys, I shan't be doing any filling. I'm just delighted the tree should survive the problem.