Hi folks
Thanks for your help. I had tried most of these ideas but they weren't working. I mentioned to someone who was helping me that when the replacement HDD was installed the DELL guy updated the BIOS. My helper thought that he could do something to the BIOS in case that was causing the problem and removed the button battery to look for some jumpers, which he couldn't find. But low and behold when the battery was replaced the computer booted from the original HDD and everything is now working perfectly.
It appears it had nothing to do with the boot files and that was why all the messing about with boot manager etc wasn't working.
So if you come across a HDD that is not booting, try removing the button battery - it might just work.