It appears there was a conflict between having a null value and first selecting the field where the max value would be 9.99. If I was to move the cursor to the far left it would allow me write longer numbers.
I have gotten around my issue by having default values in of 0.00 and using the following code to highlight the values to ensure that they are overwritten.
Please mark this as answered as I'm now happy with the above solution.
Regards,
Guy