This all depends on the rated output of your fire when run at maximum, check the rating plate on the appliance. or in the handbook that came with it.
If this is a new appliance, then it should have come with a plugtop ready fitted with the correct size of fuse inserted.
If it is rated at 3kw ( 3000 w ) then it should have a 13amp fuse,
If only 2kw ( 2000w ) then you must reduce the fuse rating down to 10amp, but again consult the documentation

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