Not a good idea to use grey twin + earth for outside electrics. it is not designed to be exposed to the elements. As far as the regs are concerned it only says a cable must be protected against damage, which leaves it open to interpretation. Good practice is for a supply to be in SWA (Steel Wired Armour) and buried deep enough so as not to be damaged by normal activity. If in a flower bed, then deeper than a spade is likely to go. Under a drive, not so deep as the ground is unlikely to be disturbed. The cable should be in a bed of soft sand to prevent sharp stones penetrating the plastic sheath. The 'guy' you are employing doesn't sound like he is following accepted good practice. Ask him if he is able to self certify his own work and if he is going to notify building control. You never said if he was protecting the supply with a RCD as is required under BS7671
