I checked the battery and it showed no signs of damage.  I have now resolved the problems.  The battery now charges when connected to AC and the keyboard works perfectly.  This confirms the problems appear to be entirely a result of the BIOS update last week to v1.24 requested by Dell.  In summary I reverted to BIOS v1.23.  These are the steps I took to solve the problem, I am sharing these as Dell appear unable or unwilling to provide any guidance without charging a fee - even when it appears it is their update that caused the problem.  I won't be buying a Dell again.  The steps took about 5 minutes: 1) Connected USB keyboard and mouse. 2) Copied personal files to a hard drive as a back-up. 3) Downloaded the previous version of BIOS (1.23) onto a USB stick from the Dell website.  It took a bit of digging to find this older version. 4) Opened BIOS using F12.   5) Navigated to BOOT options.  There is an option for flash from USB. 6) Used the 3 dot menu to navigate to the exe file save on the USB stick.   7) Due to no battery power,  the alert about inadequate power appears, and I needed to type in the dialogue box      /forceit        and press Enter.  After reverting to the previous version the task bar icons were all missing, but I clicked the blank spaces on the task bar, this opened each programme, and the icons then reappeared and remained. The battery is now charging when connected to AC and keyboard is working normally.   All personal files seem intact.
My G3 3579 required a Dell update today.  Dell has now left me with 2 problems: 1) The keyboard on the laptop lags, and then each letter is written 3 times.  (BUT, when I use a USB keyboard  there are no problems).    2) The laptop won't charge beyond 6%. I have checked all the easy options e.g. driver updates, and none work.  It seems that this problem has affected laptops since at least  2020, but Dell has still let this problem occur in 2023.   Can someone tell me what I need to do?  My product is now out of warranty and the Dell support site hasn't helped.  Thanks.
Dell's update affected keyboard and battery now won't charge beyond 6%