Dont forget living costs, (paying the mortgage, running your car, digs whilst away from home) and the unforseen additional costs that you never think about but suddenly appear and bite you on the bum it could be anythig from car insurance renewal to speeding fine to needing a new boiler in your house (if you have one that is...)
Generally a contingency of 20% is the minimum you should consider. If you don't use it, all well and good give it back to the bank and reduce your payments. If you run out of money half way through your IR it could be kind of stressful.
Dont forget an MCC as well.