Here's fun -- I worked years ago as a Quality Assurance Field Manager for two hotel chains in the US. Both had their call centers in Phoenix and the supervisor that day told me he was in the Howard Johnson room and handled a call with one of his agents - the caller had a complaint about a bad stay at a resort property in Orlando. Standard practice was to send a certicate for a free nights stay and comp the folio for the bad stay.
10 minutes after handling the complaint and ordering the send out and credit to the account he was on the other side of the wall in the Ramada section and helped an agent handle a compliant.... yup, same person - same thing, just a different hotel. The send out for the one was canceled, the charge reversed and the person banned.
In your situation you may have a fabrication and it would behove you to dig down to protect reputation. Every action has a consequence - some good, some bad and consumers should understand it is not always going to go their way.
You need proof from the consumer, doctor visits, packaging sample, costs involved for the consumer and informing them that you have a standard practice of alerting your legal firm and then inform the consumer that once you have the evidence they are supplying that you will also be contacting their local law enforcement to just let them know what is going on.
It is normally the first one - having to provide backup, packaging, doctor visit info etc that will do away with the complaint, or the second one the law firm contact that will put it to rest -- we have seen three done away with when law enforcement was going to be contacted.
I do not know about the UK but in the states a food manufacturer could take a number of ways to bring someone up on charges for false representation, defamation/slander of a company/person. If you can get the consumer to put the complaint into writing and send that to you with their evidence then if baseless, and since it is false -- bingo, you get USPS mail fraud as well.
Best to you.