the plug top fuse is there purely to protect the cable not the applience. If the cable length and size can withstand 3kw it is quite safe to usw 13amp fuseRay......Sean did not say what the EHU was to be used for and neither have any other posters on this topic.
You are assuming it is for battery charging only.
What if it is used for a 2kw heater which would blow the 5amp fuse.
Sean has posted that he has taken Woody's advice!