I suspect this is something I've done without meaning to, rather than a problem with the software itself, so I hope I'll be forgiven for looking for help here.
Working in Windows 10 Edge, I found I was typing in Strikethrough without wanting to. In truth, without knowing how to, but I soon learnt. Removed the Strikethrough but all the text inside the Britmodeller box shrank to a tiny text. The tool bars at the top remained normal. Using the text sizing tool I can change anything I typed, but nothing else. Going back to Window settings for change text size alters everything outside the Britmodeller box - the tool bars grew and the available space for Britmodeller therefore shrank. A check showed that this does not happen in Chrome on the same computer, on Android on a tablet, nor in Windows 10 Edge on another machine. Which is why I think it a local problem.
I tried closing and reopening the window. I tried closing and reopening Edge. I tried closing and restarting the machine. Nowt.