Just a note to anyone else who may be having the same problem.
After replacing the hard drive and reinstalling windows, laptop operated ok for a few days then apparaently completely failed to boot.
Power - Check, Screen - Check,
No boot messages, no error messages, just appeared to hang with nothing displayed on screen.
After a call with Dell Support, eventualy told to press/hold F8 when pressing Power On button.
Laptop then went into recovery mode. Stepped through Self-Repair options and all ok.
I was told that the reason for this was a Windows Update that had failed/corrupted something.
So if this happens to you, don't despair..... Power/F8 fixes everything (eventualy)