Hi everybody I need a bit of serious advice.

The threshold to an exterior door is seriously damaged, or rather non existent.  Entrance has a UPVC front door yet the threshold is supported by almost nothing on one side, so very bouncy.  It leaves me worried about security, and when it will actually give way.  To add complexities to this therre are airbricks under the immediate entrance, which cannot be blocked.

I just recently bought the upper flat, and clearly this threshold has seen some serious abuse and is sinking or hanging on by a thread.  Need to get this sorted very soon.

Downstairs tenants have young children, so the puschair going over it daily inceases my worry.

Hope the photographs help explain a bit better and thank you all very much in advance for the help.

Perhaps too late but...

Those don't look like air bricks, they look like metal grills covering holes. Replacing them with proper air bricks might provide enough support for the threshold.