With luck,. If you hit a competent and eager to please person. AND-if you have looked yourself at the RENFE site the correct trains nrs-I guess 2, as you need to change Sevilla.

Even then be prepared for disappointment-the2 long-dist AVE Bcn-Andalucia (nr 2 to Malaga) are often booked solid weeks advance in main hi=-season. Its even hotter as Sahara right now in Svl.

Thank you very much for your help.  The Renfe site will allow me to book the return tickets to and from Cadiz and Barcelona, there are still seats for us but at a cost is €840. If I ring Deustche Bahn (and get someone helpful) I can get these tickets for the cost of the reservation? Is there any other way to get use our Eurail passes to get to Cadiz from Barcelona?

Have you looked at the guide for how to make reservations?

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Thanks for this, the link for Spanish reservations suggests ringing Deutsche Bahn, and this seems to be on the only option I can find for booking trains within Spain having my Eurail discount applied.  Did I miss something? Is there another option?