Err, thanks Rick, but fortunately divine intervention wasn't required, this time at least!
I managed to solve it by myself, but thanks for the inputs anyway, appreciate all the help I've had on here
Just for info, I ran the SFC scan and it turned up some corruption that it couldn't repair, so I guess that's still outstanding and will need a look at now I'm running again, but didn't allow the machine to boot. I went all the way through a bcd rebuild from one of your links Boggin, and even though it completed successfully and did something on a hidden small partition of my disk that I knew nothing about, it still wouldn't allow the machine to boot. Also re-tried the full chkdsk /r and system restore (Again!) out of desperation, all with no effect.
Just before I was about to try one of Shane's full registry backups I decided to just copy the SYSTEM hive from the machine's own regback, which was more recent than my full tweaking backup, and voila, it's only done the trick!!
So now I'm back running again, only I've still no idea what the hell happened to not allow my boot record to be read and my machine not to turn on (any ideas anyone??), but it underlines the importance of doing (frequent!) registry backups to me, as I've used this method several times on both my machines to get non-running systems back on the go again, so all good for now!
Now to update and check/scan everything and then do some more backups...