I cannot get rid of black spot on my roses - any tips

Im afraid once it there, it there. The best way is to remove and burn (Dont compost affected leaves) all diseased leaves. Collect them up from the ground as the fall all the time and also clear them all up when the leaves drop in autumn.
The reason it comes back year after year is that the spores overwinter in the soil below the rose and when it rain, they get slashed back onto the new grow. Sterilse pruning tools if you have many roses so as to keep spread down.
Spray with black spray as soon as new buds/shoots appear in spring and continue aslong as spray tells you to. Hygiene is the best method.

Have you tried Roseclear 3? This is one you mix up yourself in a sprayer - not sure about the new recipe, but it used to say use double the amount if the plant already has black spot or fungus. If you start now and catch the plants early next season, plus the hygiene rules of picking up the leaves that fall, etc., you've a chance of controlling it. Some roses are more prone to black spot than others, and pruning them properly helps to keep it at bay too.

have you tried washing up liquid with a little water