self levelling compound

Yes, but another option is to fix 25mm ply or Osb to the floor to bring flush with existing, what is your finish to be?

 

Yes, but you could use a deep use leveling compound which can be used up to around 50mm, but due to it being a conservatory, I prefer to use a fibre reinforce compound, gives a stronger finish, & prime the concrete before laying compound to make sure you get a good bond.

http://www.wickes.co.uk/Setcrete-High-Performance-Floor-Levelling-Compound-20kg/p/154134

http://www.wickes.co.uk/Setcrete-High-Performance-Floor-Levelling-Primer-2-5L/p/154148

One other point, how long has the concrete been down, it need to be fully cured around 28 days before laying a leveling compound is laid.

 

Far superior response than mine, hats off to you again sir.

 

It'll only rot if you have damp rising, which as you're pouring new concrete I imagine won't happen as you've surely put a dpm down, but doesn't matter anyway, KIAB suggestion more suitable than mine.

 

There is a lot of water in concrete, which needs to escape,evaporate, if you trap it in then there is a chance that the leveling compund will lift.

Diluted pva bond primer @ 4to 1 will be ok, give floor a coat the night before & allow to dry.

As with all leveling compound it needs a helping hand to find it's level, trowel it into place,& it will then settle, & a spike roller is useful for removing trap air.

 

How about letting it dry out for a month or 2 then applying http://www.everbuild.co.uk/SikaBond-Rapid-DPM

Then mech fixing OSB?

OR lay a 1200 gauge DPM and mech fix OSB?

OR, drop the OSB and put in Hardie Backer board

 

Actually, I wouldn't use the 1200g, and I wouldn't mech fix as it would defeat the purpose of the 1200g as it would be punctured heavily with screws.

Would consider the rapid DPM and bond OSB / Hardie Backer option though