A dead area is just a dead area - maybe something happened to it that you didn't know about and it died back. So long as the area where it joined the main branch looks healthy, I wouldn't worry about it.

Thank you Bamboo.

About 3 years ago the council had to remove all mountain ash trees in my area and all surrounding areas as they were all dying from some disease

Johnjoe, are you sure they were Mountain Ash and not actual Ash trees? Mountain Ash is the common name for some Sorbus varieties - these are not susceptible to chalara fraxinea (ash dieback disease), but Ash (Fraxinus) are highly susceptible, and are being removed in large swathes of the country. If they were actually Ash trees, chalara fraxinea only infects Ash trees.

Thank you Bamboo, for your reply.
---It is what the council had told us residents many moons ago that they were mountain Ash. They used to have a red berry on them if that is any help.

Definitely Sorbus then if it had red berries - they're suffering from one of the phytophthera infections, particularly in woodland (either root rot or P. kernoviae). If your apple tree is healthy (and it looks as if it is, despite the dead branch) I wouldn't worry about it. You'll know soon enough if it succumbs to infection, and its not abnormal for a branch or two to die back without its being anything dreadful.

Thank you so much Bamboo. that is reasuring to hear.