Data Validation Error Message not Working

Hi Guys,
1st post so be gentle.
I'm currently building a simple spreadsheet that tracks time associated with Doctors consultation item numbers. Each item number runs for a specific amount of time and over the course of the day a Doctor cant exceed a certain amount of hours or else they will be audited by the government. So to stop this from happening I have created a spreadsheet that will track their time and issue a warning message once they reached a time frame or exceeded it.
So I have a column that lists each patients name on the far left and columns for each item number to the right of that. When a Doctor sees a patient they insert a 1 in the cell next to the patient and under the correct item number column. Items Numbers vary in time frames from 5 minutes to 30 minutes. At the bottom I have created a subtotal for each column the formula for that is =(SUM(B5:B163))*5 which multiplies the 1 the doctor inserts x 5 which equates to 5minutes. At the far right of the subtotals I have a "Totals" cell which lets the Doctor see the time he has accrued from the consultations, the formula for that is =SUM(B164:L164) which obviously adds up the subtotals.
The problem I'm having is with the error message which I have associated with the "Totals" cell. I have used data validation with these settings: whole number > ignore blank > greater than or equal to and minimum is 300 for 300 minutes. Error alert settings: show error alert is ticked > style is "Warning" >Title is Error > Message is You have exceeded the maximum number of minutes". At the moment I can exceed the 300 minutes and not get the warning but if i double click on the cell the message comes up. Any suggestions

Thanks in advance
Rob

Can you attach a desensitised sample of your workbook?

Here is the attachment(I think!). Thanks for your fast reply.

DV works, if I type in defined range 2 or 3 or .... different than 1 it will show Error Msg

Select the cell that contains a data validation list
On the Ribbon, click the Data tab
Click the top of the Data Validation button, to open the dialog box
In the Data Validation window, go to the Settings tab
Add a check mark to the In-cell dropdown check box
Click the OK button

Greetings,
Peter