Colour or black/white the black alwaws prints faint.

Sounds like the developer blades for the black toner is clogged. This can be fixed two ways:
The first way is the easiest, but most expensive is to buy a replacement imaging unit for $100+. (This is probally more than the original cost of the printer, and does not prevent it from happening again)
The second way is to take your existing imaging unit and open it to clean the blades that are clogged. It will take about 30 minutes, but you will learn a useful maintenance skill if you want to keep this printer. I have posted instructions in my Tips section.