Employer as always - he has given the instruction. You can provide a suitable RA and it may ameliorate the conseunces legally.

It is the employers. I used to work in steel stockholding and this was always a bug bear for drivers due to the uneven surfaces they faced. At our own sites, we supplied mobile steps to gain access and even purchased a gantry at one of the sites. The problem you have and it is one there isn't an answer for is when then have to repeat the covering / uncovering process once they reach their destination. We found many customers didn't even have any access steps to assist so we ended up retro fitting trailers with small ladders to aid the driver when climbing up/down. Multi drops also have to be considered if this is part of your business. We once had to pay a claim out when one of our drivers fell off of a trailer while NOT on our premises as it was at our insistance that he kept the load covered during his deliveries for the day. It was deemed that we made him climb up and down (we tried to argue that the customer site he delivered to should have provided something). Sorry to muddy the waters.