I have installed php in "C:\Program files (x86)\php". This is allowed by the instructions that you reference for installing php. It has worked in the past for me. Now I get the message: "Error: (-123) php extension_dir (C:\php) directory is not found. The extension directory is set correctly in php.ini. The setting is extension_dir = "C:\Program Files (x86)\php\ext"

I can access the site directly using a browser, so the IIS setup is working.

How can I fix this. (I would prefer not to move the php installation.)

you may want to check what path is set for extensions_dir in the phpinfo() output

Thanks but I've (relucantly) moved the installation to c:\php and am loathe to move it back again. It's a king sized headache to get all this working every time I make any changes.

In my case php is installed in "H:\Program Files (x86)\NuSphere\PhpED160\php54\" and guess what? it works fine and wizard has no problems with this path. You don't need to move php at all.
Just make sure you don't have any php.ini out of that directory and you don't have neither registry values nor environment variables pointing to directory with php.ini or php.exe. In this case you can have as many PHPs as you want.
Under environment variables I mean PATH and PHPRC.

Thanks. I'll try again next time I upgrade php. I'm pretty sure I was using the right php.ini but there's always the chance that there was some other one.