If the Rover battery really -is- faulty then there should be obvious symptoms like dim driving lights, engine won't turn over on the starter fast enough to 'catch' and so on.
Find someone who knows how to do a jump-lead assisted engine start and give that a try before you take the old battery out.
If the Clio battery is not physically attached to a Clio at the moment, compare all of the specifications on the label to the specs on the Rover battery before swapping them over. If volts and amps are identical, it should be okay. The "Ah" (Amp-hours) figure is more an indication of the maximum duration the battery will last at full power drain.
hth