Must've been Tesco's in Alfreton Lol! Most of the machine printers are digital nowadays and neutral B&W should be easier. When the machine printers were optical, I used to give them a piece of unexposed and developed MF film to add between the neg and light source. It usually worked 'reasonably' well if you could get an operator to understand the concept. Got to say though, the colour paper just doesn't have the range of 'proper' B&W paper.