Well, I found the checksum and the checksum seems to be fine - same as
both of my downloads that pass the binary compare.
Burned using the Mint distro and this almost booted, but then reported an
I/O error.
On the Dell I just hit "Enter" to continue, but on the HTPC it stuck there.
Something seriously wierd going on.
I am now burning to new DVD+R dual layer; probably followed by DVD-R dual
layer.
All in all this is hassle I could do without.
For context, older ISOs burned to the same stock of DVD blanks using the
same burners have so far booted fine.
I don't know if there is something wrong with the current W7 download but
this seems very unlikely as I haven't so far stumbled across anyone
complaining and as I said the checksums are O.K.
Another day I'll never see again :-(