European catfish is a predator and as such a normal fish to fish for with flyrod. Spin fishing for catfish is widespread, flyfishing not so much as there are not so many lovers of heay flyfishing after all. But as with all fish, you must fish in the right spot and at the right time. Ebro, i think that should not be a huge problem at least judging by stories of how many catfish is there. Find a spot where they are hunting and you should get results. Blind fishing should produce too, but of course where they are and if possible with such configuration, that it will be ok for flyfishing too. I think some local knowledge might be a big help with that. I have a streamer that i am developing for catfish, but am not marketing it yet.
Streamers i have put on this pade are spin and or flyrod stuff, Monster Teaser and Mini Monster Teasers are predominantely pike streamers with Monster Teaser having far more complex structure, although huchos are more than kin to smack them too. Not surprising as they are a general pattern that mimics a bunch of possible fish species, from nose carp to grayling.
Lunker Snacks were at first developed for hucho/marble fishing and general idea behind them was to merge diffrent lures and with that diffrent pray items into one lure and with that increase chances of attack. Not surprisingly, they work with other fish as well.
Have in mind that in general catfish is a night predator, although in some waters they are active during the day. Water rising etc. can wake them up too. Storms too, although waving your flyrod at that time might not be a good idea.
I am sure it could be cought on flyrod in UK and it shall, as soon as somebody shall be determined enough to discover their habits (time of hunting, where they hunt etc.) for your waters.
Btw. classic fishing for catfish with bunch of worms or liver on the bottom and a piece of paper of white foil as bite indicator has its charm when in dark you suddenly hear:"shhhhhhhhh" and that paper or foil starts to go towards the rod. I know people use live fish etc on Ebro and catfish cought there are huge, but that ain`t bad either
BlueOne