Forbidden transitions in single trapped ion systems are being considered as references for future optical frequency standards. These standards are expected to have application as highly stable and reproducible optical clocks, realizations of the metre and as optical frequency standards in their own right. This paper prescribes the work carried out at the National Physical Laboratory over recent years to use a highly forbidden <SUP>2</SUP>S<SUB>1/2</SUB> - <SUP>2</SUP>F<SUB>7/2</SUB> 467 nm electric octupole transition in a single ion of <SUP>171</SUP>Yb<SUP>+</SUP> as a frequency reference. A review of the previous measurements needed to the locate octupole transition in this 171-isotope is given and a more detailed discussion of recent work is presented. This includes spectroscopy of the octupole transition with kilohertz resolution and a direct measurement of its optical frequency. Measurements of the dynamic Stark and the quadratic Zeeman shifts on the octupole frequency are also discussed.