No you don't, although you are welcome to run it past a third party if you wish.
These days most IVA proposals are protocol and quite similar, but make sure you read them very carefully to ensure that you understand what is required.
If you are facing bankruptcy or other serious legal proceedings your IP may apply for an order suspending the proceedings. If there is such an application made you would need a solicitor to witness the affidavit but in most cases you do not require a solicitor to witness the documents.
