I've been using Patak's pastes recently. They work really well and make a great tasting curry. With the tikka massala one add coconut milk to it with a can of tomatoes to make a really delicious curry.
Son told me it's the best I've ever cooked!
They do several other types but we've not tried them yet - the balti one is next on our list. They're about £1.70 which sounds dear until you realise you'll get many portions of curry from it - unlike a jar of standard sauce which costs almost the same.
You could certainly put it in a slow cooker too - fry off the chicken until it's brown. Clever Son suggested doing it on the george foreman grill to get it nicely browned with less effort.
You can get a voucher for patak's curry pastes here