These are turning up in goods bought on-line and elsewhere. I recently ordered a replacement power supply/charger for my laptop and when I checked the plug’s fuse, was surprised to find a 13 amp fuse (5 amp is recommended) which, on closer inspection, turned out to be a fake. Tell-tale signs are its light weight and the protruding wire under the end-cap. A genuine fuse should weigh about 2.3 grams, my fake weighed about 1.6 grams.
Counterfeit 13amp Fuse, would a PAT Tester find this?

Not only illegal but dangerous – these fuses have no sand filler in the cartridge. The sand is essential in preventing an explosion if the fuse blows. Fake fuses have been known to blow the top off the plug exposing live terminals. More from the DTI.