Thanks for the the replies - I'm going to contact the firm for further details.
The debt recovery company is "BW Legal"
https://www.bwlegal.co.uk/
I've had a letter addressed to the family friend - I don't know how they got my address, but I suppose that's what they do for a living .... hunt people down :)
She obviously doesn't live here, but my address is a "care-of" address.
The original creditor is "Post Office Telecom" .... it all seems legit.