Found it! My Preciousss!!
As I thought, it was sold by RS Components. it was probably made by Abrafile, back in the day RS used to rebadge everything they sold. I'll have to take more care of it in future.
I have punches for 9, 15 and 25 way D connectors. They are great on sheet steel or aluminium up to 2mm thick. I sometimes have to cut holes in 2.7mm thick aluminium.
The difference between this tool and a needle / Swiss / warding file is that it is used for cutting a slot rather than shaping a hole. It's sort of a thick, one-ended Abrafile!
I found tile files but couldn't find one that's narrow enough. I have some of those tile saw blades, they do the job but you have to drill a hole large enough to take the end of the blade, thread it through, then fit it to the hacksaw, which can be very fiddly.
I recently moved my workshop and a very useful tool went missing. I'd like to buy another but I have no idea what it's called. I've always called it an abrafile but apparently that's something completely different. Here's a description, I hope this will ring a bell with someone...
It is...