It is not necessary to change timing gears when changing a timing chain unless they have become worn. You won't know if this is the case until you get the cover off and inspect them. It is necessary to change any chain tensioners when you change your timing chain.
To change a Vauxhall Corsa timing change you need to first remove the rocker cover, the alternator tensioner, heat shield, water pump, engine mount, and crankshaft pulley. Now that you have found the timing change, unscrew the timing chain guides, remove old chain, and replace with new chain.