I had a similar encounter in Chelmsford quite a few months back. It was a Vivastreet listing.
I didn't transfer any money and never would.
It's always the same setup. I basically spun her/him along and said I'm at the postcode now, block of flats. I said to prove you're there tell me what cars are parked outside, if they match I'll transfer the money.
I got a whole barrage of texts in the same vain as the OP.
Punters rule number 1. Never pay a deposit.
Punters rule number 2. Never pay a deposit.