Drainage into old style combined drain system

[foxalot]

The back of my house has a paved passageway on two sides with an old style combined drain in the corner (accepting both foul water from within the house and surface run off from the roof).
The ground at the back of the house slopes up and away from the house, starting with a low wall about 2 ft high, which separates the passageway from a lawn.
I want to move the wall about a foot further away from the house to make room for a porch, repave the passageway and pave part of the garden (an area about 25 ft by 10 ft).
Would it be acceptable to drain the passageway with a black plastic drain channel, which exits into the existing combined drain? Currently there is no drainage provision, and large puddles accumulate at the lowest point every time it rains. I understand I am supposed to inform the local BCO if I make a new connection into a drain, which empties into a public drainage system.
Is the proposed area of garden paving large enough to require a drainage system? The lowest point of the area I propose to pave is less than 5m from the house so it is not feasible to create a soakaway to absorb surface run off.
The underlying soil is limestone which generally drains well.

[Tony McC]

There's no need to inform the BCO of a small and relatively straightforward addition to an existing system.

All paving needs to be drained somewhere, and as the garden area slopes upwards, there's not much chance of allowing the run-off to find its own way to ground, so a simple linear channel connected into the existing MH via a trap sounds like the best idea.