Thanks for the help. I've installed ActiveTCL 8.5.18.0 as per the Python recommendation. Running from terminal:

Last login: Fri Dec 29 16:46:22 on ttys001
mkdir: /Users/Steve Margetts/.bash_sessions: Permission denied
touch: /Users/Steve Margetts/.bash_sessions/224C387E-8E74-4020-A5CC-F275EFB79EAE.historynew: No such file or directory
iMac:~ Steve Margetts$ python3.6
Python 3.6.4 (v3.6.4:d48ecebad5, Dec 18 2017, 21:07:28) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import tkinter
>>>

Did not bring up any error message. However, subsequently trying to run IDLE from terminal brought up:

Last login: Sat Dec 30 10:42:04 on ttys000
mkdir: /Users/Steve Margetts/.bash_sessions: Permission denied
touch: /Users/Steve Margetts/.bash_sessions/58F5E574-277D-4413-9B03-4B24ED9B4C21.historynew: No such file or directory
iMac:~ Steve Margetts$ python3.6 -m idlelib

 Warning: unable to create user config directory
/Users/Steve Margetts/.idlerc
 Check path and permissions.
 Exiting!

iMac:~ Steve Margetts$ 

Does this look like a permissions issue? If so, any idea what to try next? I'm logged on as an administrator on a home computer. Is this likely to be a security setting issue? Thanks again for any help - I'm a child of the GUI and the last time I coded anything was in BBC Basic!