how to deal with first floor at different levels

[crispy_wafer]

Bit of a legacy issue here, bit of a cock up during the build means that my first floor is at different levels, a section when drop measured is between 18mm and 23mm lower.  I've now finished kicking the can down the road and it's time to do something.  The flooring that was put down is the Egger or Caberdek type grey topped boards which are 22mm? It's about 30m2 to go at.

 

What options do I have to bring the levels up?  We'll be carpeting upstairs if that makes any difference to a solution.  Would something like no more ply and then a suitable SLC be the way to go?