If you don’t want the collector to contact you again give these guys a call https://www.scottishtrustdeed.co.uk/ or ask for the collector’s mailing address and tell them in writing to stop contacting you.

It's important to understand that IVA is not fixed. It is more than possible for them to go up during your arrangement. But still better to ask for debt advice when it comes to payment especially if you are confused or your not sure. There are times I have the same situation as you do.