As already been said, walk the banks with polaroids on and watch for them to show, particularly over gravel beds. I had a little trick that worked time after time for me on Yorkshire rivers and that was that if you found dace, there were usually barbel nearby, if not right under them.
Also, if fishing daylight hours check out every raft and overhanging tree. On the smaller rivers I used to fish the bigger barbel were more solitary and would often lay up under tree roots or under weed and leaf rafts.
On the subject of reels, don't ignore the Daiwa range. The Regal Z reels are very robust, very well made and have a decent size metal spool for a lot less money than the equivalent Shimano reels. Just be aware though that they are a lot bigger; for instance the Z5000 is a much bigger reel than an Aero 8000. I never looked beyond Shimano for many years, but earlier this year I picked up two Z5000 Regals for the price of one Shimano and am really impressed with them.
