It is a bit of a mystery with roach how they differ on different waters. I have frequently notice how they look different as well, different colouring mainly. One river I fish they are a sort of slate grey colour yet on another river close by they are more silvery. But they are finnicky, change what they want week to week, sometimes its small bread, sometimes large, sometimes sweetcorn or not at all.
Trying to find what they will take on what water and what day is not easy. Keep experimenting Peter, have you tried luncheon meat, I cannot try it where I fish because of a plague of eels but it is very good on some lakes I fish, small cubes on a 14 hook but bigger lumps on a 10 hook say might keep the smaller ones away and some anglers fry it a bit first to make the skin tough which also stops the small fish nibbling it. But it might attract tench and carp as well, was not a problem on the lake I fished as the carp kept well away on the far bank in the day time and the tench fed at specific times, usually about 4-6pm. The roach loved the luncheon meat though and the crucians as well. So, try some largish lumps of slightly fried luncheon meat might be worth a try.
Then there's the old favourite, tail of a big lobworm or even a whole lobworm but you never know what you might pick up but, a big roach might come along as well.
I have often said this but when small roach are shoaling around the baited area, I will fish away from it, 20/30 yds downstream or off the edge of the area on a lake, the big roach sometimes hang back looking for morsels and don't compete with the more agile and smaller fish; picked up some nice big roach like this.