Hi, thanks for your response. The private key I have is 52 characters and begins with the letter 'K'.

The message above the key itself states

"Key createdAt is in UTC format as specified by ISO 8601
#   e.g: 2011-12-31T16:42:00Z . The century, 'T' and 'Z' are mandatory"

When I try to sweep it into Electrum a message appears that says:

"No inputs found. (Note that inputs need to be confirmed)"

I'm not sure whether this indicates that there is no BTC 'attached' to the private key, or whether the format of the key is incompatible with the wallet.