Mercurial executable not found (mercurial 5.7.1)

I have just set up TeamCity on a new machine (TeamCity Professional 2020.2.3 (build 86002)) and downloaded the latest Mercurial (5.7.1) with e TortoiseHg overlay and am getting the above error when trying to test a VCS connection on the server. The help says that it should work out of the box and not to edit the config unless asked to by support. The hg.exe has been installed in C:\Program Files\Mercurial. Should I actually be editing the config to get this to work ? I tried adding this line teamcity.hg.customServerHgPathWhitelist=C\:\\Program Files\\Mercurial; as per the help via the Diagnostics--> Internal Properties page just in case, but still get the same error after restarting the server. Any help ?!