2000 1.6 Focus Powerloss

Hi guys, new to this so any help will be greatly appreciated.

i have a '00 mk1 3dr Focus which i've personally done a lot of work on myself,

and also had a lot of work done by garages.

recently it's been losing power,

I almost though it was the clutch on the way out as the revs climbed and it didn't go anywhere.

the engine is noisier than usual(not the sweet sound of the zetec), as if there's a big exhaust on it.

after a short drive today i checked the engine and noticed that the rocker cover over one of the cams was hot but not the other, could it be a fault with the zetec system? That only one cam is working instead of two?

All theories are welcome,

Thank you,

Ritchie.

Edited: also, it will only rev to roughly 4100 rpm whilst driving, and the cat on the exhaust started rattling 2 weeks ago but i haven't gotten around to fixing it yet as it passed the NCT like that, maybe that has something to do with it...

Updated: 12/11/2015

I had to take it to the garage for a new battery when my Mondeo failed to start, was bad going down but coming back it was absolutely perfect, so problem solved... or not solved, still bugs me that i can't figure out what the hell is wrong with it.