How to safely screw down chipboard subfloor?

[redhexagonal]

So my chipboard hallway subfloor is dipped by 7mm and I want to level it and I’ve been advised its easiest to use a self levelling compound.

Problem is the boards aren’t secured too well, they move a bit and don’t have enough screws, and the screws are only 50mm and are coming loose. So I need to screw the boards down before using the SLC. At this point I have a fear that I will screw through a water pipe/cable/gas pipe, which all run under this floor somewhere. I can see them going in on the other side of the hallway wall (as the floor is up next door, you can see the gas pipe, leccy and white water pipe going under the wall to the hall in the pic. Unfortunately I'm not sure where they go next)

What is the safest way to screw the boards down without a DIY disaster?

 

[RandomGrinch]

Hi,

If the boards are not well secured and move around a bit, why not take them up as you have done next door?
Shim up the boards to level them, no need for SLC, and you have the benefit of knowing exactly where the cables and pipes are.
IMHO, that would be the safest option!

 

[redhexagonal]

Hi,

If the boards are not well secured and move around a bit, why not take them up as you have done next door?
Shim up the boards to level them, no need for SLC, and you have the benefit of knowing exactly where the cables and pipes are.
IMHO, that would be the safest option!

That would involve sawing along the edge of the wall as the boards went under the wall. The wall would then not be supported as much (its sort of half on a joist) and would probably need noggins.( Although previously the entire wall was floating above the floor and moved when I pushed it!)
I was going to get a builder to do that and level the joists, but I can’t find one for love nor money (the builder who finished the other room won’t come back, complicated) and and I’m not confident to do that myself. Which is why I thought I’d bodge it and use a SLC but then realised the floor isnt fixed down properly…. Going round in circles here!

 

[RandomGrinch]

To be fair, it looks as if your pipes and wire have been laid at safe depths
One of my lead gas pipes sneaks up over a joist, notched just below a floorboard - that was an interesting discovery!
Does the gap at the top right of your image extend into the hallway?
If so, why not stick your phone in and take a pic?