You can't say for certain why your roses have black spot & there isn't much you can do about it unfortunately. Are your roses being shaded by the oaks? That's another problem. If your roses don't receive at least 6 hours of sun per day, they are prone to many other problems, black spot being one.

And in any case the Black spot which is on the Oaks is not the same as the one on the roses. Every plant has its own specific type and cross infection does not occur except between family members.
As Bathgate says, shade from the Oaks could be a problem, as could their roots depriving the roses of food and water. Strongly growing plants can fight off infections better than struggling ones.

Use one part cow's milk mixed with two parts water in a sprayer, weekly, spraying thoroughly and until run off - treats black spot on roses.

Treating 'black spot' is futile until you find out why your roses are getting it in the first place. The roses might recover once or twice, but will eventually give up altogether.

Also make sure you pick up all the affected rose leaves as the fungus will overwinter in the soil and affect next years leaves.

My rose garden is an octagon in the centre of the lawn and so they get plenty of sunshine . Thanks to all who answered my question,I will try the milk treatment.

So you're surrounded by Holme Oak trees that are dropping leaves with black spot on your roses, yet your roses get plenty of sun? The oaks can reach 65 feet.