The procedure is probably laid out in some BS and/or IEC standard. (Much of the information contained in the NEC is found in various standards in Europe) If you are unable to find any standard or other information, it can be that the "intuitive" approach is used.
I'm not going to pretend that I know anything about motor sizing, but I did think about one thing that might help you. (Separate circuits for the three motors would seem to be sensible.)
10 hp ~ 7 kW ==> 10A @ 400V 3ph
I expect that you know the power factor and inrush current? The smallest standard 3-ph circuit is 16A (using 2.5 mm2) as this is the smallest IEC 60309 plug. For a 16A type C breaker, the maximum current should stay below 80A.
As for the larger motor: Calculate it as per the NEC and see what numbers you come up with. The laws of physics are the same on both side of the pond.
(Although motors are often sized in kW rather than hp on this side.)
[This message has been edited by C-H (edited 01-17-2003).]