IDNet tell me that they have found a bug in the software on the new router that went into service earlier in the week. This, coupled with BT's work on the new central may have added to the stale session problems that some users have encountered. Cisco have been informed of the bug.
The router has just been re-booted (15:12) following which users should find that normal service is restored. Anyone still experiencing problems should contact IDNet by phone.
Edit: Date added to subject for clarity