Thanks, Neil.

Hmm. I didn't download the mapsigpatch for the unlocking operation on the Mac, but perhaps it is only for PCs. I will try and find a friend with a PC, but if I have to do that every time it may reduce my use of the iMac.

I understand that the original file is probably unlocked and overwritten on the Mac. Do you think that will have happened, and without mapsipatch?

You probably have a PC. What is the BAK file you refer to? Is it the original locked file, and does bak refer to a .bak suffix i.e. file type or does it just refer to a copy of the original (or perhaps the original itself)?

Finally, in the gimgunlock folder (for Macs) that I downloaded, there are two files, "gimgunlock" (which is the one I used to (try and) unlock the .img file), and "gimgxor", which I have not used yet. Do you know what that latter file is for?