Is the Forward button greyed out for every e-mail that you read? Or just some?

For all emails

Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled".

If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

If the issue still exists when all add-ons are disabled, then restart Windows in Safe Mode with Networking and Thunderbird with add-ons disabled.

Restart the operating system (Windows or OSX), in Safe Mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX.

Then see if Thunderbird works better. If it does, then another program or process is interfering with Thunderbird when Windows is running normally.

Thanks

Unfortunately when I statrt TB in safe mode the mail toolbar reverts to default and the buttons I added are not present on it

I can't reproduce that. My Forward button (yes, mine is on the main toolbar) does change in appearance if I have more than one message selected, since it needs me to decide if I should forward them all individually, or all at once as multiple attachments to a single new email message. But the little drop-down list button remains active to give me this choice.

Does a right-click offer you a working forwarding facility?

Just found my problem ... and I feel a bit of a fool :)

Turns out there are two "Forward" buttons that I could add to the toolbar when I customised it & I picked the wrong one. I assume the one I picked is for navigating within threads rather than for forwarding emails. Anyway fixed now - image below to explain what I mean

Thanks for the help & apologies for time wasted

I thought that might have been the problem, but you said the button was on the mail toolbar, when it is actually on the application toolbar, so that threw me off.