@Stu After selecting the markups, find the Units dropdown in the Properties Toolbar. From there, you'll be able to switch the units from mm to m. This example shows a couple lengths, perimeter, and polylength all able to be adjusted at once.
Thanks Ryan and can I now to set that as my default?
I would recommend placing a length measurement. Use the Properties Toolbar to find Units and switch to m. You can also set the Font Size here. Add this customized markup to a Tool Set and double-click the icon until it looks like the example (switching it to Properties Mode). Using that tool will set each length to meters and indicated font size.
thanks Ryan