600D not detecting Magic Lantern

[tupp]

I will find a 4Gb card tomorrow and give it a go

Any 4GB, 8GB, 16GB or 32GB SDHC card should work.

Each file copied in turn on to the card then deleted from the computer before checksum performed. I'm not sure how I can verify that these results are valid??

All looks copasetic.  If you eliminated all traces of the firmware files from your computer and if the files on the card were links or were corrupted, then the md5sums would be off or some error message would have appeared.

[tupp]

I'm running Win7 64bit, and explorer is set to "show hidden files"
[snip]
I'm pretty sure that this is where the problem lies, but have no idea what to do about it.

Try disabling "show hidden files" in Explorer, and see if the files are visible on the card through the card reader.  If they are not visible, then the files have the "hidden" attribute set and that condition is very possibly involved in your problem.