OK, I've read through all of the links that were kindly provided to me now an have made some progress:
I changed the bit with the arrow and that has resolved the NAN error and now retains the best score between sessions which is great.
But now the best score continually increases along with each point scored. For example, your best score is 10. You play again and it tells you your best score was 10 but then, as you play and score more points, the best score increases by that amount of points:
Now, I know I need to convert something to 'string' in the area marked with a question mark but I'm getting rather confused!
If someone could perhaps tell me what I need to change (and to what) in the Scoring area to make it work correctly that would be really great.