H18 refers to Hidden 18, the regulations that govern Driver's hours.
On the 12th December 1988 3 trains collided outside Clapham Junction. Anthony Hidden QC investigated on behalf of the government and came up with 93 recommendations!! Number 18 refers to driver's overtime, requiring it to be monitored so that driver's are not wroking excessive hours.
Therefore Guidance in the Safety Critical Work Regulations 1994 is based upon the Hidden Recommendation 18 from the Clapham Junction railway incident of 1988.
The recommendations are: a maximum of 12 hours per duty; 12 hours rest between duties; a maximum of 72 hours per week; and a maximum of 13 consecutive duties
I assume that you will have some responsiblity for ensuring that systems are in place to make sure this regs are complied with.