Patio drainage channel (?)

[Jamie C]

Hi all,

Thanks in advance for any help.

I have a split level garden where the patio is at house height and there is a lower lawn area separated by a brick retaining wall.

There is some sinking on the patio and after lifting some pavers have found a channel cut into the concrete below running away from the house. There appears to be a sand base on the concrete with dabs of mortar on the slabs.

I am not sure how to proceed in levelling the patio as a) I'm not sure what the channel is and b) how I should relay a sub base without it eventually crumbling away into the channel.

Pics hopefully help.

Thanks!