Precision 7560, no power, bad motherboard?

Warranty January 30, 2022 to January 30, 2025. This was working perfectly before I went away for 10 days. It was then powered off and stored in a cupboard.

On return it started normally, plugged into mains charger via the co-ax. I received a notification that Dell Update needed to install 3 updates (I already did the latest BIOS update, before going away). OK'd that and carried on. I then received a low battery power notification to connect a charger. Apparently I had removed the mains input from the charger before going away. Plugged that in, and the machine shut down, with absolutely no signs of life since - no LED, no fans nothing. 

Thinking perhaps CMOS had been corrupted, I tried the reset procedure. No change. Done the flea power discharge etc. 

I have since stripped the machine, removed and a few hours later, replaced the CMOS battery. All back together with no problems, and still dead. I have 2 180w chargers and 1 240w, all correct. I am fairly sure there is a motherboard short which temporarily shuts them down. The plugs and power coax socket look perfect. USB-C Thunderbolt charging also does nothing. It mostly leads a pampered desktop life.

Can I send the board for testing and repair/or exchange (in UK) or what? It's a complicated laptop and a fair amount of work which I'd prefer to DIY for cost reasons. I am suspect this is going to be horribly expensive if a Dell tech does the whole strip/repair/rebuild. 3 year warranty cover ended in February this year. It had already had a mobo replacement the previous year.