Is Peterborough your local county court?
A lot of cases are being automatically transferred to St Helens for telephone hearings, which is not good for these cases. It may be that they are now getting overwhelmed with these cases and they are transferring them back to the defendants local courts, as they should have been in the first place.
Or, it could be because the sum claimed is more significant than a few hundred £, it requires a hearing in person and will be longer than standard.
You will receive a directions order from Peterborough County Court in due course.
I suggest you do not mislay ANY court documents as that is how people end up with default CCJs because of missed information. Are you sure that the missing letter was identical to this one?