I work for a rival company, OCS Retail Support, that is exactly the same job. Like you said it is a zero hours contract. They send you, by email, available shifts and you choose which ones you want.
You make your own way to customer sites and there is usually at least a couple per week that will be close by. At my company, the first 70 miles round trip is not covered as expenses, and the mileage rate after that is diabolical. Parking is not paid for, so you have to find free parking usually quite a distance from the customer site. This means you have to walk back to your car late at night/early in the morning, often in rough areas or pay for parking for the duration of your shift, which is 1-2 hours wages gone.
The work is boring, repetitive, unvaried but it is easy, although it can be quite fidgety. Shifts are usually between 6-10 hours and at terrible hours.
It is an odd job to do, but it is extremely flexible and you can make a decent supplementary income from it.