On the Banana Pi M7 Standard Case how is the CPU heatsink metal block secured please ?
Are you referring to this case?
The CPU dissipates heat by directly contacting the case through a thermal pad.
There are assembly videos on YouTube, you can check them out.
Banana Pi BPI-M7 RK3588 open source SBC Housing complete installation video vs Raspberry Pi 5
I have heat stressed the unit using stress --cpu 8 and found its does not disappate heat well enough. I have bought an internal heatsink and fan unit and need to remove the SOC heatsink metal block in order to install the heatsink fan unit.
From what I understand, the case and cooler seem to be incompatible. The case is likely a one-piece design, so you might not be able to remove it.
Oh I am removing it 
I need to install the fan and heatsink unit, and most like then drill some holes in the top of the case and then through an extra heatsink on the top by the looks of it.
I would just like to know how it is fixed in, which might make it easier.
If is a glue then I can find the right solvent.
The top space of this casing is rather limited. If you need to install a fan, you may need to conduct a careful test before making a decision.