I found a work around!
It seems that the 255 character limit is only applied when pulling text from a cell using a formula. You can achieve more characters (in my case around 350) by simply copying the text from the cell and pasting into the text box.
This of course is a manual solution which just wouldn't do. But a simple bit if VBA code can automate this. You may also need to adjust the formatting of the text box as it resets itself. This can also be applied using VBA.
This is my code - adapt to suit:
Hope that can help someone facing a similar issue
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