On Mon, 20 Apr 2015 19:48:47 +0100, GB
wrote:

On 20/04/2015 19:07, MM wrote:
In my rummage box

Have you tried a little card next to the letterbox:

"Please ring the bell after you put anything through the letter box.
Thanks."

That would be effective, and "simplest possible".

Each bell push has a laser printed label (had so for months) saying
"Please ring BOTH bells!" You have to realise that not everyone can
read nowadays. They can recognise house numbers and somehow they work
out which street or road they're on, and that's about as far as their
academic experience ever went. My postie (well, one of them; I get
several different ones) can't even tell the difference between 7 and
5, because sometimes I get the neigbour's mail and she gets mine.

MM