To be honest investing your sinking fund is a dead duck at the moment. The current interest rates being offered on the market at the moment are tiny, compare that with the inflation rate and DIRT you will be lucky to have the same value in your sinking fund in 5 years as you do today.
The main thing you need to focus on is guranteeing the money in these uncertain times. I would guard against sticking your money into a long term investment. Having easy and quick access to at least a part of the fund is essential to ensuring the developement runs smoothly and that your money is protected. The government gurantee expires at the end of this year but is likely to be extended, but just to clarify, if you were to enter into a ten year bond with say BOI tomorrow, your money would be guranteed for the term of the investment. ie your money is guranteed for 10 years by the government.
However on the flip side interest rates are going up and are likely to increase again in the near future, by locking your money in long term means you lose out on potentialy greater returns when the interest rates go up.
My advice, choose a 14 day or 30 day demand account for a substantial portion, and lock in the balance in a 1 to 2 year deposit account in order to earn some return. It wont make you a mint but at least you have accessible cash on hand should the developement need it or the banks become unsteady.