I had my “Slow the Flow” waterbutts fitted a couple of weeks ago and yesterday found that two of them had blocked outflow pipes.
It seems that the fitters are not being as careful as they should be while doing the drilling to fit the pipes. I found quite a large amount of plastic shavings floating on the surface of the water. Some of these shavings had blocked the outflow pipes.
I have written to Southern Water about this but, in the meantime, I suggest that you check yours and fish out any green plastic shaving you find on the surface of the water and clear any that have found their way into the outlet pipes.