Are there any basic principles about cooking curries with tomatoes, slow-cooked meat and yoghurt that I should go with?
Generally – not always, but usually – don’t use yoghurt if you’re using tomatoes, they tend not to play well together.
But for a weekday, slow-cooked curry don’t bother with spices and similar malarkey. Get a jar of whichever Pataks paste appeals (the paste, not the runnier sauce), follow the instructions on the back until you get to the ‘simmer’ stage, bung it in the slow cooker and later on, Bob’s your bream.
If you want an alternative that doesn’t involve the slow cooker but still doesn’t have you following the whole recipe from scratch yourself, pick up one of the Spice Tailor packets. The instructions are simple, the spices you need are there and they don’t take too long to knock together; good stuff for a midweek evening.
I say all this as a curry fan who likes the whole process of doing it himself from scratch; but things like Pataks pastes and Spice Tailor packets exist for a reason and I’m happy to use either when it’s appropriate.