I've had the occasional barbel trotting the float/isotope combination but a lot more chub. I started to take it more seriously several years ago on the Dorset Stour where the last hour or so into dusk can be their switch on time. One the age old dodges is to allow a few more yards of trot even if you've lost sight of the float, in my case it was a few revolutions of the centrepin. It can work!
I decided to trot it a bit more effectively and well into dark rather than switch to a static or freelined bait and had a fair amount of success especially with chub. It pays to know the swim your fishing very well as things "change" after dark!
It can liven up and extend the somewhat harder daylight sessions.
These were two nice chub that fell to the extra couple of revolutions of the centrepin routine