try turning on the contacts pane. F9 when in the write window turns it on and off.

To add to Matt's suggestion, once the contact sidebar is open select all the contacts you want to BCC by holding the control key and clicking them. At the bottom of the contact sidebar are buttons labeled Add To. Pick the button that meets your needs.

That sorted my problem - partly - BUT, when 'Forwarding' mail to these multiple contacts, I want to delete the box at the top, titled 'Forwarded message' which includes the original senders details. Why can't I just highlight the whole box and hit delete?? I can partly delete some of the details this way, but I want to remove the whole box. Why does Thunderbird make simple things like this so problematic??

Select the table that contains the information (those are the lines that are only visible when the message is edited.

Click in the message so the lanes are visible then from the menu Format > table > delete table.

Thanks Matt. I've been using another email client for over 10 years and have just dumped it for Tbird, so I'm slowly getting to grips with its little idiosycricies!