As mentioned, the Wansbeck estuary or on the beach at the mouth, been a bit hit and miss this year though for me.
Seaton Sluice harbour mouth used to be brilliant and chocker with flatties until about the mid 1980's, not sure what happened (sewage spill maybe) but they all vanished almost overnight and I've not had anything bar the odd crab out of there ever since.
I've you walk through the Tyne Pedestrian tunnel there's a jetty on the south side near it that used to be great, not sure what it's called but used to fish there with my brother and never failled to get some nice flatties there every trip.
Could try Druridge bay, some canny sandbars and channels on the beach there just north of Cresswell in the area between the 1st and 2nd carparks (ie opposite the farm with the big pond at the roadside). Can see the channel and sandbar in the pic here, this was as the tide was dropping last weekend: