Reading your posts has made me further reflect on the effect of hedgehogs.
In my particular situation with a combination of heavy leaf fall in Autumn and moss on the roof, the hedgehogs do get a fairly strong but qualified recommendation. Obviously the comments below may not apply to those with houses unaffected by lots of large trees nearby.
Since they have been installed we have noticed quite a lot of moss rolls down the roof and drops on the patio/paths at certain times of the year. We didn't notice this before hedgehogs were added to the gutters. Sitting in the garden in the summer, I have been amused to see that pigeons are very helpful in removing moss from the roof. They appear to be finding insects underneath and they are the main culprits in prising off the blobs of moss.
From memory, prior to 2012, the moss rolled into the gutters and, together with rotting leaves and sprouting plants, formed an evil-smelling sludge which occasionally blocked the downpipes, particularly the bends at the top. I had to dismantle them to rinse out the sludge. Now, moss rolling down the roof usually bounces off the hedgehog and drops to the ground below. So a disadvantage is - lots of moss on the patio at certain times of the year. This is easy to sweep up.
After a year or two, the hedgehogs here build up a layer of moss on top which, ironically means all rolling moss bounces off down to the ground.
So this is the reason, I suspect that, since fitting, we haven't had a serious overflow from the gutters nor blocked downpipes. But lots of loose moss on the patio. It seems sensible maintenance to remove the hedgehogs and flick the gunge off, say every two years. I do this myself on the lower gutters and can see that there is virtually no sludge underneath, even though the moss on the top is quite thick.
Isn't it fascinating, when one is a bit of a geek, retired and in lockdown, to consider all the implications of guttering?
regards
Howard