Countup time and display in a cell

[kersplash]

In the attached example there are cells in Column K & L that have start and end dates.

This is what I have - the formula in Column M only displays the elapsed time between the 2 dates if both dates are completed. This takes into account public holidays and only includes time between the hours of 9 and 5 on weekdays.

This is what I would like - If there is a date in Column K and no date in Column L, I want Column M to display the elapsed time in hours and minutes from the Column K date (only including working hours and exclude public holidays). This should update every minute while there is no date in Column L. Once there is a date in Column L the timer for that row should cease changing the value in Column M.