Hi @johnbunney
Perhaps try a full power refresh and check if that resolves the problem.
I realize that you tried to update the BIOS but you said that it failed.
Turn off the laptop (if on) and disconnect the charger (if connected).
Disconnect the main battery (you don't have to remove it, just disconnect it) and the RTC coin cell battery from the motherboard.
Here's the [https://dl.dell.com/topicspdf/vostro-14-5415-laptop_owners-manual3_en-us.pdf|owner manual] that may help, although you've already opened it so probably not much. Go to .17 and p.31 to view the procedures to disconnect the battery and the RTC coin cell battery.
When the RTC battery is removed, measure its voltage. If it is <2.5V DC or if it is >4-5 years old, replace it.
''Press and hold the laptop's Power button for a full 30 seconds and then release it.''
Reconnect the RTC coin cell battery, reconnect the main battery, reassemble the laptop, connect the charger and check if it boots OK.
If it does there may be a message saying the date and time are incorrect. This is normal as the BIOS has been reset by the above procedure. Once the date and time have been corrected the message won't appear the next time the laptop is started.
If still no good, check if testing the memory using this free [https://www.memtest86.com/download.htm|memory test program] works. The idea is not that you're testing the memory per se, but as the program is a standalone program i.e. it doesn't need Windows to be loaded, it checks if the BIOS boots OK (from a USB anyway) and that most of the hardware functions are working as a lot are being used by the program. Not the AHCI controller though that is used for the M.2 SSD