Mark D wrote:

Looking at replacing the manky fence panels soon, and
during my searches I found the common size fence panel is
40mm deep. My concrete posts have a 50mm recess.

Surely this will lead to rattling in the wind, but I can't
find any spacers or brackets, or any mention of such
things.
https://amazon.co.uk/Fence-Panel-Grips/dp/B00YQ6MKQG

https://www.ebay.co.uk/itm/124271152661