If you have appointed an IP to represent you, it is essential that you send them all correspondence from your creditors, so that they can ensure they are working with the most up to date balances.
I did not send any letters up to my creditors meeting as there was no point as each creditor contacted the IP with how much I owed. I was advised that up to two weeks after the IVA was approved I may still get phonecalls and letters and it was only after that 2 weeks that I should contact my IP firm and they will deal with it for me.