I do love a strongly worded letter of complaint and have always received cash/cheque when complaining to Southern in the past. Maybe they've changed their rules, but If they dare send me a voucher they'll be sent a detailed diagram of exactly where they can stick it!!!
I've never had the pleasure of receiving a submitting a 'Delay Repay' claim as I rarely by a season ticket. What are the vouchers redeemable against? (I'm guessing it's going to be train travel). It's a bit of a piss take they don't send you cash.
EDIT - Ooops just seen earlier answer