Dell E6430u i7 Laptop Won't POST

I’ve recently acquired a used Dell Latitude E6430u i7 laptop. The laptop is in very good condition but will not POST.

When the power button is pressed and released, the mute light and caps lock flash on very briefly and the fan spins for about 1 second. Nothing else happens, no sounds, nothing on screen. If I then hold the power button to force shut down, the fan spins for a second or so before shut down occurs.

Having read around various articles including Dell support, I have tried the following to make the laptop POST:

1. Start up with ‘D’ button held down to test the LCD. The test procedure runs OK, but then when the laptop tries to start afterwards, there is no change to what I described above

2. Start up with ‘FN’ button held down to enter diagnostics. No success, no change to what I described above

3. I have tried removing battery, holding power button to remove flea power then going for forced ePSA as described in Dell support. No success, no change to what I described above

4. I have tried changing the memory sticks for memory of the same specification from another laptop that I know is working. No success, no change to what I described above

5. I have tried the Dell BIOS recovery tool. No success, no change to what I described above

6. I have opened the laptop case and disconnected the CMOS, removed charge and then attempted to start as described in Dell support. No success, no change to what I described above

So I am now running out of ideas for things to try. At present, the mSATA SSD is not fitted, but I don’t think that this would prevent a successful POST to a point where the screen would display a message. The laptop is a high end machine in very good condition and it would be a shame to discard. Does anyone have any further suggestions for things to try? My only other thought is that it might be a BIOS problem or maybe a replacement motherboard is needed. Any advice would be gratefully received.