Hot is too cold in winter

I have a Grohe thermostat mixer tap where I can adjust the temperature and the flow rate separately. I winter I can't get it to run hot enough, even though I have set the temp as high as I can. It is not a problem with the hot water because I can get scolding hot out of the sink. It only happens on really cold days when the cold water is ice cold. I need to find a way to get to let less cold in and more hot, but I am not sure how.

I am not sure of the exact model but the insides look similar to this http://www.grohe-addon.com/additionalcatalog/files/tpi/95/t95453131-01.pdf

Can anybody tell me how to adjust this so I can get more hot water and less cold?

If you have access to the pipework you could put a ballafix on the cold and reduce the flow with that

If its thermostatic you shouldnt need a balofix on it to regulate the flow. the thermostat will still mix it to the temp required. More likely the thermostat is nacked.