I understand that in summer the service may operate at weekends.
The best way from Lagos to Seville at weekends is either to take the train to Faro and get the alsa.es service from there to Seville
or
Train from Lagos to Vila Real Sta. Antonio
Ferry over to Ayamonte
Walk through Ayamonte to the bus station and then Bus from Ayamonte to Seville.
For more info see:
Cubsur's
www.algarvebus.info
See his latest comment
"TRAVEL BETWEEN THE ALGARVE AND SPAIN"
My recent blog on this:
http://barlaventostravelblog.blogspot.com/2007/04/traveling-from-faro-to-seville-by-bus.html
General Travel info:
http://www.solpraiaferias.com/index.php?content=cheg_en