My next project is laying a patio, and I want to include a ground
socket for a rotary washing line. It'll be concreted in, so won't be
easy to replace later without damaging the slabs.

Is there a standard size for washing line ground sockets? I hope the
patio will outlive the washing line.

Also, are there any recommendations for good sockets? I don't mind
spending a bit more on a decent metal socket if it'll last better than
a plastic one.