every cable is different & every installation method is different, for example a 6mm twin and earth cable touching insulation will not be big enough to carry 40amps, but a 4mm twin and earth cable clipped on a wall will carry 40amps , although you then need to start factoring in the length of the run, how many cables are bunched together, ambient temperature. Its a minefield !!!!!
going by ntsty's table, that particular 6mm cable would be big enough for a cable length of nearly 40m at 40amps, not taking into account any other derating factors other than volt drop !