Re #23248 Sanitise the # character

Re #23248

MantidPlot/src/ProjectRecovery.cpp

@@ -126,7 +126,7 @@ getRecoveryFolderCheckpoints(const std::string &recoveryFolderPath) {
 
 std::string removeInvalidFilenameChars(std::string s) {
   // NTFS is most restrictive, so blacklist on this
-  std::string blacklistChars{":*?<>|/\"\\"};
+  std::string blacklistChars{"#:*?<>|/\"\\"};
   boost::remove_erase_if(s, boost::is_any_of(blacklistChars));
   return s;
 }