Finally managed to get the two I was after.
I've been playing about with the original address all day until I found a link that was always up.
I then installed a widget to refresh the page every 10 seconds and eventually got in.
However, I have noticed over the last 24 hours they only seem to open up the portal to buy tickets at certain times and for a limited period.
As someone else said on her it's best to register first to save time on checkout.
Hints;
Use:
https://www.marcaentradas.com/eventos/estadio-de-la-cartuja-entradas
Once there you should see a link to the match that always seems to be active (but doesn't necessarily have tickets).
Try to copy and paste the original link (not paste and go) then add a number to the end: 1 was working now appears to be 5
When pre-registering on the ticket site you need a Spanish NI number (or rather their equivalent) its 8 numbers and 2 letters - 12345678AB will do.
Install the extension "easy auto refresh" on Chrome to save you manually refreshing.
Good luck fellow Bears - hope to see you in Seville.