Thank you very much. These are just the examples which help me a lot.Yes, happy ending is the one you want.
As regards their general use:
end is a verb and a noun - to end/ an end
ending is a noun - an ending
We seem to use ending with an adjective more - so I've reached the end of the book BUT its ending was very good.
An end can also be a goal - e.g. I did my homework with the end of improving my english. Ending cannot be used here.
Most of the time end is correct except in the context of describing a film/story with an adjective.
One exception would be that we say someone came to a nasty end meaning they had an unpleasant death which obviously involves an adj combination.
I'm afraid these are just examples I can think of - I've never heard of there being definative rules. Wait for someone else.