Porkpal, see my last post - in France (Menton) we have hot summers and sandy soil, so pretty similar, I think. DA roses like lots of water and food because they rebloom so frequently.
In addition to rose food, we use a combination of Viresco products (Humate (on planting and once or twice a year thereafter), Viresco Soluble (once a year) and Viresco Foliar (supposed to apply it once a month but we don't use it that frequently). All are biological products which in combination help the plants grow much bigger roots, take up more nutrients and resist disease. They certainly make a difference and in spite of the poor soil the roses are really healthy - no black spot, no rust, very few aphids. And they're not expensive. Here's a link to the UK Viresco website, although I believe the company originated in the USA so you might find a better source.
https://www.viresco-uk.com/inf...
Even without the Viresco products, as Neal says I'm sure you would get a wonderful first flush. In France we sometimes have to prune twice rather than just once a year because the plants grow so vigorously.
