Can anyone please advise correct syntax to change the Bot strength/level in the console for Wolfenstein Enemy Territory please.

Is this your server? you cant be just a player on a server and mess with bot settings you have to change them in the server config. So if its your server then here you go....

/bot difficulty 4

Difficulty ranges from 0 to 6 (0 is easy, 6 is hard).

If you change difficulty, bots' properties will be adjusted (reaction time, field of view, aim error, view distance, ect.

Advanced users can adjust bot properties for each difficulty level in the file omni-bot/et/scripts/goals/goal_difficulty.gm.