Hi, I'll reply in more detail a bit later - thanks for the posts.
I only have my system reserved partition with a drive letter so that EasyBCD could locate it when I used it to move the bootmgr back there from the C drive. I don't usually have a letter assigned. I'll remove it at some stage soon. (Kevin, removing the drive letter won't cause anything strange to happen - at least it doesn't in my system. It doesn't make it inoperable doing that, just means Windows won't write to it, basically).
The problem is that my system boots so much more slowly. When I did a search for 'bootmgr' on the PC, there were files in both the C drive and the Z drive. That can't be right, surely??