Update - I sent the text and got a call back - it was genuine.
I was offered half price for 12 months on my original package - Big Kahuna, 100mb, phone line and weekend calls, 2x old Tivo, £28 (retained a legacy Mates Rates discount in that price). It didn't seem possible to get the kit upgraded with that deal unfortunately, or the free Movies offer (yes I may be expecting a lot, but only going on what others have reported as linked to above - a common feature in the really best deals seems to he putting in your cancellation and ordering a new service from a competitor to transfer your phone number which seems to open up even better outbound offers - I didn't do this last stage, which may be my downfall
).