Years ago, Sainsburys first introduced little specialist food sections in some of their stores. It was a sort of gourmet section. They had in there, a small jar of piccallili, about 9cm high by about 6 cm diameter. It did not have the original manufacturers details on, as it was made for Sainsbury's. It was the best I have ever tasted-I have tasted many since and they are not a patch on this one. When the product was withdrawn, I asked Sainsbury's who the original supplier was. Sainsbury's refused to tell me. I am just wondering if anyone out there has any idea?
As you mentioned, it was probably made for them by a piccallili manufacturer. As there surely can't be that many companies making this product, why not contact the larger ones and ask? They are probably more likely to part with the information (especially as it may get them a new customer) than Sainsburys were.