After a lot of faffing about, the only way I could get this SearXNG container to route through Gluetun successfully was to KEEP the 8080 port (I changed other containers so that this one could have that port instead), and to use the "--network=container:GluetunVPN" extra parameter, setting Network type to None (the gluetun container dropdown on Network would not work). So, it was a pain, but it works now.