Adding times

I'm trying to add the following list of times:
09:30 + 09:00 + 09:00 + 04:00 + 08:30 + 08:30 + 06:00 + 05:00 + 05:00 + 07:00 + 08:00 + 05:00 + 04:00 + 07:30

Using the following formula:
=IF(INT(ROUND(SUM(L4:L70)/0.04166667,2)) = 0, "", INT(ROUND(SUM(L4:L70)/0.04166667,2)) &":" & IF(MINUTE(MOD(SUM(L4:L70),1)) < 10, "0" & MINUTE(MOD(SUM(L4:L70),1)), MINUTE(MOD(SUM(L4:L70),1))))

The problem lies in the part of the function marked in bold. This function is working perfectly with all/most other combinations of times.

Hopefully someone can help me.

Thanks,
Imelda.