I bought a 10 year old car (Citroen C3 )
Sorry to say it, but that was your first mistake, buying a French car, especially a 10 year old one.
Your second mistake was buying an automatic, they're not appropriate for British town driving.
It's an automatic but it's not auto in the sense I've become used to.
It's not an automatic, it's a semi-automatic. It has a manual gearbox and gear changes are made by electronics activating solenoids to move the gear levers. CLICKY for more info on semi automatic gearboxes
Quote from the above: "Despite superficial similarity to other automated transmissions, semi-automatic transmissions differ significantly in internal operation and driver's 'feel'"
I can feel it change gear and it just doesn't drive smoothly at all.
It's normal for semi-automatics. There's nothing you can do about it, though as the Dr says, try changing your driving style to see if the computer controlling the gearbox adapts. Barring that, set the car on fire and get another one