Wow, nice trip! Never seen Stonehenge in the snow before...

I agree that Salisbury is worth a visit in its own right, it's one of those towns that has a particularly nice feel to it.

As for the 2 for 1 offers, we should add that to one of the FAQs. If you're just in London, then you can still take advantage of them by making sure that, if you buy a Travelcard (paper one-day transport pass), that you buy it from a National Rail outlet, and not a TfL outlet.