Hi Steve
As jerry says, chewy's 3 hr base is a good reference for a base which will taste right without needing excessive cooking during the final dish. Key to getting a good balance into the base with sufficient sweetness is the duration of the first cook, ie getting the onions properly "floppy". Tasting them at this point will give you a clue how much bitterness remains.
In cooking the final dish don't be tempted to stir too much once the base sauce goes in, twice or three times is enough and if the base "catches" a little (gets thicker) at the base of the pan, this also helps develop sweetness and depth of flavour. Final tip, cook the curry on a nice high heat and if you find it thickens too quickly due to evaporation, offset this by adding more water to your base before you start (ie. next time start with a runnier base). The aim should be to cook on a high heat for 5 mins after you add the last of the base. Cooking on a nice high heat also contributes to the sauce "catching" a little on the base of the pan.
These effects are reasonably subtle, but important to getting a good flavour. They won't help you much if the flavour of your base is off to start with, so make sure your foundations are strong first.