Some times i will bulk the shot for roach but i find the bite comes quicker with a slower fall, might sound a bit stupid because the bulk rig gets down quicker but think about it this way:
if you are using a positive bulk rig with the bulk 12 inches away from the hook then when you put the rig in the bulk drops down in a strait line dragging you droppers and hook with it,at the moment there will be an arc in the line below the float.
With a string of smaller shot the line will fall straighter.
A little test to try is to get a couple of pieces of cotton or line and attach a float to one end and a hook to the other end,put them on the kitchen table then put a curve in one and keep the other straight, pull both hooks at the same time away from the float and watch how long it takes to move the floats.Then think about how many roach would have spit the hook out in the time it takes to move the float on the curved rig.
When roach are feeding a bit iffy they dart into the feed grab at one item at a time and dart straight back out,hence quick bites.When they are feeding well they stay in the feed eating other food items moving slower,thats when you get those slow bites that are unmissable.
Might be a load of rubbish mate but the guy who told me about it was a guy called Steve Webb (still my hero), he used to fish for Shakespeare superteam on the Avon and has forgot more about roach behaviour than most of us will ever know.
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;) 
Small silvers or any bonus fish?