Hmmm - Ron Sproat isn't credited with a script until Ep #94. It certainly seems odd that he could have been working on the show for 6 weeks before he wrote a script - especially given how tight DC was when it came to the money. To have paid Sproat for that length of time to get up to speed seems way out of character for DC. WAY OUT OF CHARACTER!
For example, Francis Swann was already writing scripts almost as soon as Wallace brought him on board.
As for Wallace, there are a few interviews with him and with Robert Costello in which it's made clear that Wallace was around nearly the whole first year of DS. My favorite Wallace interview was published in the DS Files series of books back in the '80s - and in that one it's also confirmed that he was around for the creation of Barnabas.