I'd be inclined to remove the ivy, if possible, seems a terrible waste of a south facing wall. Ivy's all right in dark corners where nowt else will grow. Alternatively, do try a climbing rose - that would probably work better than Clematis, but you will have to keep the ivy trimmed back flat against the wall to give the rose light and air. Also ensure you keep the rose watered during dry spells if the ivy roots are extensive.