ive been working all day with my L5 and its never cut out on me yet, never had it running long enough.
dont forget the cut off is a safety device that cuts it off when the heater gets to hot! or put it another way, to stop heater packing up, plus dont forget its gas!!!!!
all you need to do is stop the flow of water until the heater turns off then start all over again, it resets it self, if you get in a habbit of doing it each 10 minutes you will have no problem.
plus i know my L5 has a daily runing cost of just under £1.30 a day!