Send them to your former IP and they should be able to deal with it. Regardless you are protected but you would want to ensure that your credit file is updated and that these letters are not causing a problem on that score.
We completed our IVA about 4years ago and still receive these .today I received one from Santander who took over Alliance and Leicester with an outstanding balance they said I owed but this was part of the IVA . I thought this was against the law for them to continue after completion .
Also you might want to ring the creditors and resend them a copy of your completion certificate and ask them to update their records. There are always a few that slip through the net.
I'd be tempted to send a copy of the completion certificate (assuming youi have this) with a copy of their letter informing them the IVA is complete and therefore no further correspondence is required.
IVA final payment left the bank on the 26th January 2013...looking forward to a debt free future.