where to put extra mcb on consumer unit?

I have heard it said that some manufaturers include "Diversity" in the design of the consumer unit. I believe this is typically 80%. This means that although the unit is rated at, say, 100 amps the maximum load is assumed to be unlikely to rise above 80 amps for any appreciable length of time. One reason for keeping the larger circuit breakers closer to the switch is that, on the occasions the total load is at or above it's 80% maximum, the length of bus-bar conductor under full or overload condition is at its shortest and therefore less likely to cause overheating to the other circuit breakers.

Steve
November 2009

It's not necessary to put the highest rated mcb closest on the DIN rail to the main switch fuse unit. The physical position in that sense is irrelevant. The difference in resistance due to a difference in length of conductor of 15 or 20 mm is negligible. Each mcb can be fed by a separate piece of panel flex cable or a common busbar (strip of copper which is supplied with the consumer unit) can be used to connect them.
More important for you to notice is that some mcbs are between the switch fuse unit and the RCD and others are the other side of the RCD. The neutrals relating to those circuits are wired to 2 separate neutral blocks (i.e. those protected by the RCD v those not protected by the RCD).

J
November 2009

Ideally, yes.
Why? Conductors should be kept as short as possible, electricity takes the easiest path, etc

Adam
November 2009