//Even if they walked away and carried on ignoring the social distancing they could be asked or instructed to leave.//
Yes they could. And it wouldn't need a Covid Marshal to do that. The store manager could do so and he or she would have to call the police if they refused to leave.
I see no role for Covid Marshals in shops or stores. In fact I see no role for them at all. Looking at the Hertfordshire press article, it seems neither does anybody else, short of the usual Local Authority bureau-babble:
//In its description of the marshals’ duties, the contract notice reads: “Provide practical support to aid and encourage compliance, such as dedicated staff in public areas, business support, or support for individuals.”//
Individuals don't need support to comply with requirements that are not enforceable by law; they will comply with them if they want to and won't if they don't. Businesses have been dealing with Covid for more than a year; they are well aware of their responsibilities and how to discharge them.
"Covid Marshals" are part of a concerted campaign to make the public continue to believe that they are subject to control. The control people's everyday lives should be diminishing and no "Marshals" are required to do that.