Problem solved,,,

[#1]

But not sure why it was there in the first place.
After getting my PC back yesterday with the new power supply I noticed that one of the internal hard drives was absent. On opening the case I could see that the power cable was not connect so rectified it. Starting up the PC it would not get past the Windows Logo. Even Starting up in Safe Mode would get stuck while going through the list of files. I disconnected the same hard drive again but still it refused to start up so put the original W7 Pro 64 DVD in and booted from that and did a repair which solved the issue.
Since then I haven't reconnected the hard drive again and it wasn't showing any signs of trouble before replacing the PSU. Of course the PSU could have borked the HDD at the same time as it failed but, if that wasn't the case - I won't know till I take the tower to pieces again during the week - what else could have caused the issue?

[#2]

I have observed many times that too many hard drives can cause boot problems or crashes.  This can be alleviated by using a PSU with a greater power output capacity but even then some drives can cause problems.  I presume the problematic drives require more power than the average. 

[#3]

My previous PSU was 650W and the new one is 750W and more than ample especially as all external hard drives have their own power supply.
I certainly agree many boot and other issues can be caused by hard drive issues and can be difficult to trace.
I probably won't be able to open up the case till later this week but once I have and reconnected the offending drive again I'll be able to see if it still causes problems and run diagnostics on it if so.

[#4]

Don't be "conned" by PSU ratings.
I read a recent article where they tested various cheap 500W-850W PSUs and all but one failed at far less than their ratings.
Their conclusion was that a decent brand name 400W PSU far out performed the cheap "pluck a figure out of the air" Chinese PSUs.
Of course, if you have a decent branded PSU then the above is not of any concern
http://www.tomshardware.com/reviews/low-cost-psu-pc-power-supply,2862.html

[#5]

it would be worth checking the hard drive boot sequence in the bios also did the dodgy hard drive have an operating system on at some point

[#6]

@'Mav' - does your PC BIOS have a setting for the hard drives, where you can specify a delay period (on power up) as to when each drive is allowed to spin up and have it's interface enabled ?
I'm wondering (as a long shot !),  whether you could use successively longer delays (ending with the OS boot drive),  so that the peak power supply load of all the drives currently starting simultaneously, can be made to stagger in time, and therefore reduce that peak current demand.
Many of the motherboards I use can specify a delay, but have never had the need to use it, and less likely to now I use SSDs.

[#7]

@oldjiim The boot sequence was wrong when I check the BIOS after the first failure. But putting that right didn't make a difference. No, I don't think the offending drive did have the OS on before but don't quote me on that
My understanding of the boot sequence is that each drive is tested for the OS in turn until it finds one or an error is reported.  The SSD with the OS was higher up (or should that be lower down) the priority list than the HDD in question anyway.
@purleigh I needed to enter BIOS several times yesterday to check various settings and never noticed any delay period setting.