Well. Phew. We've made some significant progress on the 'lastminute.com top secret hotels' front.
Because I made a big fuss on twitter (and here), I managed to get a lovely lady from the lastminute.com VIP desk to look into our problem and it has been more or less sorted out.
At least she actually listened to / read / understood the problem! While I'm happy that it has been adequately dealt with, the misleading information correct and we have been compensated (with a room upgrade!), I'm still somewhat peeved by how hard it was to get the same simple message through via their standard support channels.
I dread to think what would have happened if I hadn't used social media to complain, though.
Apparently they are going to investigate the awful asssistance we received - I shall report back with the outcome (if we are kept updated, can't remember if they said they would or not)!