The 300mm to the side is recommended, but can be reduced if the material is non combustible and is for using the appliance and for clearances for pot handles etc
The 50mm either side is to do with the height of the wall units from the Worktop
If the units are more than 450mm above the Worktop they can be vertically in line with the edges of the hob
If the wall units are lower than 450mm from the Worktop the edge of the units must be set back 50mm (on either side) from the vertical line,
So if you have a 500mm wide hob and the units are high the units will be 500mm apart, if the units are lower than 450mm they will need to be 50+500+50 apart, the reasoning being if they are lower there is a higher risk of them getting burnt
Does that make sense?????????