Could it be a counterfeit sub-standard fuse from the far East? We are frequently hearing of products with fake approval marks entering the UK market. I recall warnings about exploding plug fuses with false approvals. BTW I had a look at some of the fuses in plugs around the house & lying in my tool trays, they all had the BSI Kitemark & ASTA approval marks, no other approvals.
"The majority of the certified plugs in the UK have been tested in our Laboratory. We now offer our own certification scheme and the mark as an alternative product certification mark. "