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What self levelling compound do you use? All the stuff I looked at from a packet can only do about 12mm. Anything more and you need to add something to it.
I still say that try and figure out what is there from plans, as getting a structural engineer is going to be pricy as any worth his salt will want to expose to confirm what is actually there and any asbestos tests to as depending of age, they put asbestos in everything. Proof is Central Hill blocks of flats in crystal palace.
lynx-
All the stuff I looked at from a packet can only do about 12mm
Fosroc make a couple of products that are self levelling that can go deeper than 12mm in a single pour. I think one can actually do 50mm but last time I used it (~ 5 years ago) it retailed at about £120 for a 25kg bag so it's spendy.
The other option is to do a 12mm pour then once completely dry and cured do another and keep building up layers. This also gets pricey and is time consuming, plus down the line there is no guarantee that the layers won't break apart and there is no comeback because you are going against manufacturers recommended usage instructions. There is a misconception that self levelling cementicious products are designed to level a floor that is on the piss, they're not they're really designed to eliminate hollows in a floor that is relatively level.
Sometimes in my line of work you have to explain to a client why their idea is stupid then accept you're going to be ignored and use your experience to make the best of a bad job. Because of this I'm pretty happy to do a 12mm self levelling pour then "top it up" but my level of concern rises exponentially quite soon after this.
Bobbo
A structural engineer is usually a good place to start.
So the floor slopes downhill to a step which then goes down into the hallway? I vaguely remember a drawing you did explaining this in more detail but the specifics escape me.
Self levelling could be made to work if you shutter off the top of the step however it has the potentially to be quite a thick pour and therefore require a lot of self levelling compound. You also have to remember that there is a maximum thickness you can pour in one go. While it is possible to do multiple pours to bring it up to thickness it can cause problems.
It may be worth considering getting someone to lay a cementicious screed. Good outfits are fast and will get it flat first time. However you will need to stay off it for a period of time until dry. Also you will need to consider doing something with the step to make sure any steps are not too big and if there are multiple steps they MUST be even otherwise it is considered a trip hazard.