This isn't my area of expertise, so I'll simply provide you with one important quote, from the DirectGov website:
"Let your IP know if your financial situation gets worse, for example, if you lose your job. They may be able to get your creditors to agree to lower monthly payments".
It's not my field either Joe. However, these firms contacting you must be making money somewhere along the line - and that can only be from you. I would suggest you stick with your current arrangement. If you are experiencing difficulties then do as Chris suggests and contact your IP and let then know the situation exactly.
I agree entirely - do not get sucked in to having anything to do with these firms.
If it is not possible to come to an acceptable arrangement within your IVA then it will have to be failed. You will then have to look at other ways to cope with your problems - one of which is possibly bankruptcy. You can get free advice on dealing with your debts from your local CAB or by ringing CCCS.