I assume you have the Regular version of the 30CDi (not the combi or the system) with a separate pump.
Follow the balancing instructions and set the pump so you get as near to 20C as you can or slightly less. Then balance the rads to the same temperature. You will find that the temperature differentials across the boiler and rads will have changed when you get to the last rad. So you will have to go round several times to get them all approximately the same.
If necessary, adjust the pump speed to maintain the correct differential.
The important thing is that the differentials are, if possible, the same. This then ensures that they are all getting their fair share of the heat.
You say the rads are slightly oversized; how were the sizes determined? They may be more oversized than you think.