how can i fix my worcester bosch combi boiler 280 RSF?

When i turn on the hot tap i only get teppid water not hot.this is worse with the shower,if i run the cold tap it does help a bit.The central heating is working fine.If i run the heating at the same time it makes a small differance but not alot.Can you please help me,i am able to change any part that might be faulty. I have a Worcester Bosch 280 RSF wall mounted combination boiler.