Hi Dave,
Glad to hear you had a first time success.
1) Doubling up. Try it and see what you think. I manage to make a small double portion for the wife and me using 500 ml of base in a wok. I wouldn't like to put much more in and I think CA's Jalfrezi calls for 300 ml per portion.
Remember though that this is basically stir fried food and you often have to raise and lower the temperatures quite quickly while cooking.
2) Just add more chilli powder and/or use a hotter chilli powder. With a jalfrezi you have the option to use hotter fresh chillis as well.
3) Salt and sugar. Just add less next time. I use just over a quarter teaspoon of salt per portion. I don't usually add sugar to savoury curries.
More dishes - check out CA's bhuna, madras, chicken tikka masalla etc.
Cheers,
Paul