You will need to confirm whether or not the existing FCU in on the ring final or if it is a spur itself. If it is not a spur then yes, take a spur from the supply side of the FCU. If the existing FCU is a spur itself then no. However if you tell us what the existing spur is suppling, fuse rating, appliance rating and rating of waste disposal unit it may be possible to wire it in a different configuration to that explained above. More info req.
I presume the waste disposal is in the kitchen so it will be notifiable work (special location).
PS I am not an electrician so wait for further replies before doing anything! Method of safe isolation, 17ed regs, RCD's etc.
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