How can I fix my Worcester Bosch 240 combi boiler hot water problem?

i have a worcester bosch 240 combi boiler aprox 10 years old. my heating works fine but my hot water seems to only give luke warm water when tap is fully open. If i close tap slightly to lower water speed and hence increase temperature burner turns off.
This problem is gradually getting worse.

is there anything I can purchase to service the unit myself or is this one for the professionals