Reading through various (negative) reviews in the East and West Midlands (my main punting areas) it seems that a few unscrupulous WG's have cottoned on to a deposit scam.
I think you are at least lucky to get away with a relatively small £20, and this should be a salutary warning for anyone tempted to send a deposit.
You might just get a bit of flack from some on here for sending the deposit, but I at least thank you for the warning regarding this particular WG who now goes straight onto my blacklist. If a WG scams another punter in this way, then that means she will look to scam me in whatever way she can, even if she doesn't ask for a deposit.
I wish you better luck with your next punting attempt.