how do i turn down the water temperature on my worcester 24i boiler???

everytime i try to use the shower i either get scalded or i freeze, there is no hot water temp control on the boiler, when i turn the temp comtrol on the shower it goes cold, the only way i can have a shower at a good temp is if i run the hot tap on my bath or sink. i'm ready for takin an axe to it. my landlord is useless so have decided to try n fix it myself. thank you to anyone who does answer this problem

p.s i'm in a 3rd floor flat and cannot turn the water pressure up have already tired but from the looks of it the pipes running up are all 15mm (dont know if that is the right size or not)