Ahh... one I haven't read...
but I do think that it does help to have the 'sound' of it fit the story... I will def. have to get a copy of Difference Engine now... :D
While I like the Difference Engine a lot and it's use of Victorian language is brilliant, I would say that it's more important for the author to find his own 'voice' whether it's Dickensian or not, otherwise the entire affair is going to sound contrived. To me, Difference Engine sounds like Gibson/Sterling rather than Dickens.
One more thought is that in early Victorian era writers Dickens, etc, are much more readable than late Victorian writers (excluding greats like Conrad, Doyle or Kipling) , as they dressed up their prose with lots of flourish and ornamentation.
I don't recall Dickens using terms like "very flash". Gibson overused "flash" to the point that, were I to ever use "flash" in a book, I would have to use it consciously as a nod to Gibson, or not at all.
If I want to get a vague idea of how people actually spoke back then, I read first hand accounts written by non-writers. Then again overuse of unfamiliar terms can necessitate a glossary and make the work largely unreadable.
Do you mean this anthology? I picked that up a while ago and only read ta few stories, but the language didn't strike me as particularly Dickensian.
It's important to get the tone right in period pieces, even in SP, but I don't think it necessary to attempt to emulate a writer of the time. For one thing, Victorian writers rarely used two words when fiftenn would do, for another it gets in the way of your own voice.