Camray compact Limit Light?

I have a Camray Compact and the Limit light keeps coming on. It did this a few times late last summer. It was then serviced. Now during this cold snap it has started happening again. The heating and hot water is set to come on morning and eve and we sometimes leave it running through the day. During the coldest few days it came on every morning and had to be reset. It resets fine. It occasionally does it during the day as well. It is not happening as frequently now the weather is warming up.

For what reason does the Limit light come on?

I have told my Landlord he has done nothing yet, I would like to know why the boiler is behaving like this.