I feel a little sheepish, having fixed the problem tonight. I was selecting Plate Solve from the Tools Menu not the "Solve and sync" button in the scope controls.
I have learned a lot about Sharpcap plate solving along the way...
1. The core engine to install is Astrometric.Net/ansvr. Astrotortilla (et al) are not required unless you want to use it as a stand alone front end. Sharpcap is an adequate front end to centre a DSO for an image capture session.
2. Astrometric.Net appears to be an http hosted plate solving engine. The configuration can be accessed at http://127.0.0.1:8080/api/config. You can also access access the config via Astrotortilla. I am not sure whether or not the configuration parameters are common across applications. I assume that the parameters exposed in the Sharpcap Plate Solve settings are the only ones to worry about. The ansvr server has to be started to access the config via http, using Internet Explorer as some of the more recent web browsers don't seem to work. I don't think that it needs to be started for Sharpcap. I believe that it accesses the Astrometry.Net engine directly.
3. When you install Astrotortilla is supposed install Cygwin (at C:\\Cywin), Astrometry.Net and the index files. It all worked for my a few years but my recent attempts all failed. I eventually installed Astrometry.Net only, it's what Sharpcap actually uses. It installs Cygwin in C:\Users\username\AppData\Local\cygwin_ansvr\bin (bash.exe etc). The index files are stored in C:\Users\username\AppData\Local\cygwin_ansvr\usr\share\astrometry\data. Not ideal, in my mind, but fighting it can lead you to a world of pain. When I later installed Astrotortilla, I installed only Astrotortilla itself and configured it to use the cygwin_ansvr bash.exe, which seems to work. This avoids having multiple copies of Cygwin and index files saving much confusion if you reinstall.
4. If the Index Downloader is aborted, it can leave the Data folder (containing the index files) renamed Data.Save, causing things to no longer work. The file that was being downloaded will exist in the Data folder with 0 file length. The Downloader only checks for existing file names, not sizes, so the incomplete file won't be replaced until the 0 length file deleted.
Edited by Rac19, 14 May 2021 - 06:36 PM.