Could anybody please tell me if it's possible to over cook a base.
I don't mean as in burn it I mean is leaving it the longer the better or will this impair the flavour or would it even improve it?
EDIT: I am referring to the initial melting of the onion stage?
From my experience, once you've reached the oil separation stage (after blending) a base doesn't improve with additional cooking, and I find that you can get a 'stale' smell reminiscent of a school kitchen (well my old school's kitchen, anyway) if you cook your base too long. It's the classic aroma of overcooked veg. (It does seem to improve a little if left to cool and stand overnight, though.)
Including the melting, blending and oil separation stages, I'd say around two hours is the maximum cooking time for a base.
But like so much of this lark, it's a personal thing. Others may have different experiences and opinions, which is what this forum is all about...