Heritage Open Days opening thousands of doors to the public

Over a couple of weeks in September, several thousand buildings will open their doors to the public.
This is the Heritage Open Days, a fortnight of open days that’s essentially the same thing as Open House London, but (largely) happens outside London.
All are free, some need booking in advance, otherwise, just turn up on the day.
The list below is of the venues in the counties around London (and a couple inside) that look interesting places to visit and are generally not open to the public