If you click an an empty space next to the Reply button, you can choose Customize to customize the icon set. You probably have the Reply button selected, instead of the Smart Reply button. Change it and you're all set!

So if I click empty space then customise I then get a screen of various icons I can add to my Toolbar so what do I then do. Mervyn

There's a Smart Reply button in the list of icons that you can add. Add it to the right sport by dragging it there and remove the old Reply button by dragging it from the toolbar to the iconset.

No smart reply button on list at all only the reply to list that I already have.

Do you have the same version of Thunderbird on both computers? I seem to remember a bug was recently 'fixed' to change the reply behavior for replying to all or to the list.

Will need to check that tonight. Thanks as I think one has a beta version. Mervyn

Thank you for your excellent help as this home computer that had the problem was on the beta channel but all others on normal and changing this off beta allows the reply button to work properly. Mervyn

This issue was probably triggered by bug 77304 - Cannot reply to From: when Reply-To: field appears in header. This behavior will change in Thunderbird 52, so for now you're set by switching off beta, but this will come soon in a theater near you.

Thank you at least that answers my problem.

The release notes for Thunderbird 52 state that

"When replying to a mailing list, reply will be sent to address in From header ignoring Reply-to header"

This appears to have fixed one bug by introducing another. The correct behaviour should be that the Reply-to is honoured by default, but the user has the option to override it.