Patio Drainage to Land Drain and Sewer?

[Bad_chili]

Hi all,
Having a bit of difficulty working out how to do my patio drainage so some help would be greatly appreciated!
Firstly, my plan is to run a ditch along the bottom of the slope of the patio then and around to a point where it can be drained to my sewer. In the ditch I will be placing perforated pipe and gravel. Would I be right in my thinking that the ditch will have to be dug at a slope towards the drainage point?
Secondly, the drainage point will be a redundant clay pipe that runs to an inspection/junction point for my combined sewerage. It was previously a kitchen sink drain that was relocated when then sinks was moved and now just has a cover over it. I plan to dig around the pipe at the point that suits me cut the pipe and then use a suitable adapter to connect to the land drain. I understand that I will also need to incorporate a 's trap'? Does this sound like this will work? Is there anything I'm not taking into consideration?
Many thanks