Hi Nigel and welcome to the forum
You ar rightly worried about this as the iva would be dommed to fail if you cannot afford the payments. Please do not sign and send back anything you are not 100% happy with as it needs to be sustainable for 5 years.
I started off with an iva provider and got to the stage you are at now when I realised the advice I was being given was not all correct, I then found the forum and changed to Melanie Giles and have never been happier and feel I have got the right iva that will be concluded successfully in 60 months.
It was daunting to change IP's at that stage but I know I did the correct thing.
Consult another IP asap for another opinion, you can always look at
www.iva.com for reviews of IP's done by their clients.
Good luck
Be positive & look after yourself, there are more important things in life than debts....
Best Wishes
Emma x