ECU and cover removal

My Clio has a missfire not on a specific cylinder and I am told the ECU may need to be repaired. Does anyone know how to remove the black steel cover on the ECU which is mounted behind the battery please. Is there a speical tool required? Thanks
 
I wouldn't go near the Ecu until everything else ignition related has been checked.. Out of interest who told you the Ecu needed repairing?
 
I found the suggestion on another site. Originally the cambelt broke and I replaced it and some bent valves. I used the renault tools to time the cams and it seemed to be ok when I restated it. By the time I replaced the fan belt and started it again the missfire appeared. I checked all wires etc and cannot see anything out of place. I checked compression and all cylinders are good. One sugestion was to take the cover off the ECU and check the wiring from the connector. One site suggests the ECU is at fault based on the fault code. The cover wil not budge no matter what I do.
 
highly unlikely to be that given the amount of other work that has gone on. Surely must be timing related in some way?