well I am just starting one and I think it will take around 4.5 yrs to pay all my debts off.
Has anyone ever used one? if so which one and are they a good idea?
Is it possible to have a second income (casual work) going into a different account and have this as extra spending money or will I have to the DMP company about this?
I presume you are talking about some sort of debt management company. Avoid any company who charges you money to sort out your debts - they become another creditor to add to your list and money which could be used to reduce your debt goes to them!
There are charities out there who will assist you free of charge. Your first call should be to Citizens Advice or look at www.cccs.co.uk.