was the stereo addressed to you?
if so, if you are sent something in the post it is legally yours.
if a mail order company sends you something in error you are under no obligation to pay for it or send it back
This is from the CAB site:
"Other additional rights you have when you make a distance sale include the right:
to be given the name and address of the trader
to be given the price of the goods, including taxes and delivery charges
to receive the goods within 30 days
to keep or dispose of any goods you receive but have not ordered. If a seller demands payment for goods you haven't ordered, you don't have to pay for them or return them."
i only know this because we were sent a lawnmower we didn't order - we thought it was a wedding gift and used it, then the company tried to make us pay for it...until we quoted them this...
you might be in luck (so long as you get your dressing table)