No letter here and my most recent bill still has the 151 number on it.
I really can't see the benefits of being one of the few telecoms companies without a shortcode for cust service access. It's not like they have to pay BT interconnect fees for customer dialling 151 from their lines.
Plus a few times in the past the 0800 number was unavailable for several hours but 151 kept on going.
The only possible explanation is they are getting ready for putting the Freedom brand under the 'normal' Ntl and want all advertising to carry the 0800 052 2000 number.