triton bezique 1 cold and warm cant get it hot????

I cannot get my shower hot the most i get out of it is luke warm when the heat setting button is pressed on 2 which is the highest and the tempreture gauge has been turned round to 3 o'clock but the water flow is not so good if i turn it back to 11 o'clock the water is freezing and it was fine up until a few days ago.....any advice welcome please.
Thanks