Hi Kcon,I remember catching quite a few small tench purely by accident while fishing for trout,several years back.I was swinging a team of small buzzers round on the breeze at the time,Arthur Cove style.
Mind you the water I was fishing (a small weedy reservoir on the outskirts of Edinburgh) was stuffed full of tench at that time. I believe the angling club who were leasing it in those days, drained it and netted all the coarse fish out.....

dont ask me why.
Apparently there were some tench in the 4-5lb class in amongst them but I was only hooking wee fellies,12ozs up to a pound or so.
Scrappy wee guys though and good fun on light fly gear.You might have to attract them into a swim first on a less prolific water..?
Good luck...