Ariston Microgenus 23 MFFI boiler - only tepid water?

Hi, we are in rented accomodation - landlord is a nightmare. I was wondering if anyone had any ideas why this boiler is not giving us hot water anymore. Through the summer months it has been fine (in fact blissful - i like hot showers) however since the weather has turned so has the temperature of the showers. At best it is tepid, sometimes bearly that. I don't know what to do really.

The boiler was not installed by CORGI reg plumber I am fairly sure (landlord cuts all corners). I know legally he requires to provide a safety certificate however I don't wish to get into a big fight with him. I haven't the time at the moment.

So... any ideas what is causing this problem? I will call in a plumber if it is likely to be a fault and take it off the rent, however I am nervous that the problem is in fact just that this is an appalling boiler and no mending will help.

Any advise would be super as I really am beginning to lose my mind!