aceman's sausage axiom notwithstanding, if you are interested in cheap caffs, you could do worse than:

Soho greasy spoon: Bar Bruno (corner of Wardour and Peter Street).
Covent Garden greasy spoon: Can't remember its name but there's a great Italian run greasy spoon on Macklin Street just off Drury Lane.

However, I too like a decent sausage, so don't go to the above places that often. You might consider:

Oxford street midscale: The Eagle Bar & Diner (Rathbone Place)
Covent Garden midscale: Joe Allen (Exeter St) - certainly on a Sunday and I think a Saturday too.
Piccadilly Posh: The Wolseley