Hi Toby,
The IP's fees are agreed in advance, and the creditors can say if they think they are too high, and lower them.
If you were to go into an IVA, then your payments will be assessed on your disposable income, and a dividend agreed. These payments can go up if you have a payrise or earn overtime or receive a bonus. This has the advantage of paying more back to your creditors, but you will never pay more back than you originally owed, plus the IP's fees.
You can make a full and final offer during the IVA and a meeting will be convened with your creditors to see if they will accept it. The offers usually need to be as near to the original dividend as possible and you must have a good reason for proposing it - money gifted to you by a third party is a popular one.
Approach as many IP's as you like -
www.iva.com is a good place to check out a lot of companies, and reviews. They will give you free and impartial advice as to all the options open to you and the best way forward for your circumstances. Make sure that you are happy with the company that you choose.
If you own your own house, be aware that you will probably have to try and realise some of the equity in the 4th year to pay across to the IVA. If you are unable to, or there is no equity, then your IVA will usually continue for a further 12 months.