Looking through the options and help files didn't help, and the no-doubt correct advice from the #mIRC channel I was too stupid to translate into action.
I figured it out finally. So I'm posting it here in case it might help someone else with the same problem in future.
(Digressing, I want to say thank you to all the kind people who have posted stuff online to help clueless twits like me.)
Anyway here is my take on the answer:
I figured it out finally. For reference, the answer I
needed was something like:
"Go to the Tools -> Favorites -> Organize Favorites
(or Alt+J or click the 'Favorites' button (little heart
picture, third from the left))
Then, in the 'mIRC Favorties' window select the channel
you are interested in from the list (use the 'Add' to add
a channel if it isn't there)
Once you have selected the channel click the 'Edit'
button. In the resulting 'Edit Channel' window, click on
the 'Join on connect' tickbox. Click 'OK'. Then check
in the "Enable join on connect" tickbox. Click 'OK'
This has worked for me. See the mIRC website for advice
by people who actually know what they are talking about:
(mIRC automatically joining channels.)