Seems there is a lot of confusion about this matter, which considering the conflicting replies I got even upto and including the generally excellent CEO's office staff isn't surprising.
- In my case we had an old NTL-branded Samsung STB.
- For the upgrade it was free 500GB TiVo (no install/activation/unit charge), extra £6.50 per month for M+ package and £3 for TiVo service.
- If the household wants another STB it's an additional £6.50 per box regardless of whether it's a V HD, V+ HD or 500GB TiVo, which means choosing the TiVo makes the most sense since you're getting more for your money.
- The £3 TiVo service charge applies to the whole household, regardless of whether you have one box or several... think of it as like the Sky Sports channels, where one subscription fee covers all STBs in the household.