Approaching would-be Vendors

Hi,

I was wondering if anyone could offer any advice as to how I should go about approaching the owners of a particular property that I am interested in buying.

The property is not "on the market" and the owners at this stage are not aware that I am interested.

I am keen to buy their unit (which is in the same block of units as one I already own) and would like to make the best first impression I can, either by phone call or by a written letter.

Any thoughts or suggestions would be greatly appreciated.

Thanks,
Ian