Kerak,
The situation you describe looks much more like an employment than a self employment, what kind of tax arrangements are in place?
I would agree with Rainydays sentiments that over reliance on any one client is a bad thing despite how financially beneficial it may be. You are currently doing 30-40 hours per week, what is your weekly capacity, if you have an additional say ten hours why not seek other clients to utilise your additional capacity, remember when this contract comes to an end you will still have to maintain your business, and over reliance on one company for such a long period may damage your ability to do so.
With regard to the requirement for a contract, what is the industry norm ? do you want to be tied into a contract with this company, this situation is defined by your need to maintain other clients if you wish to remain self employed.
Regards,
Ikeano