Thanks for the help and advice. The problem was not,  as I thought,  the power cable;  nor the Data cable.  Rather it was the SATA data port on the board.  Swapping the data cable to a different port,  resulted in everything springing into life.  Not sure what caused the original data port to stop working,  but there was a spare ... so all is well.    
I wonder if anyone can help ? My Dell Inspiron desktop (purchased just over three years ago) failed a few days ago. The DVD drive and Power light flash in normal boot sequence but the SATA hard disk does not spin up. SupportAssist reports that no hard disk is detected . I removed the HDD, inserted it into a USB/SATA container, connected this to another computer ... and could access the disk and data correctly without problem from that PC. The diags on that computer reported no drive errors and no faults on the disk. Replaced the HDD in the original machine, rechecked all of the cables were seated well; machine still fails to boot and BIOS reports no drive other than the DVD. The BIOS logs show no changes since 2020.  There have been no hardware changes since purchase. I am very puzzled, any suggestions ? Possibly a fault in one of the cables unlikely though that sounds ?
Dell Inspiron fails to find SATA hard disk