Ford Fiesta overheating problem

Could try flushing cooling system , radiator may be partially blocked or needing replaced . Might be worth rechecking timing was set up correctly when cambelt was replaced also .

How do u flush the system?

If you had the thermostat and water pump replaced they would drained most of the cooling system and should have noticed if there was excess rust / muck in system .Normally you would loosen rad. drain and flush with a hose . If no drain tap , loosen bottom hose .You can get radiator flush from the likes of Halfords if needed .Take it the rad coolant fan is cutting in when you say drops back to normal ? you sometimes can hear it cutting in if stationary . Many a time flushing doesn't cure a partially blocked rad and it needs replaced . Usually can tell by the weight of removed rad if you have to go that far , if it's choked or not .Remember and refill with antifreeze if you drain system .Could still be timing out though .