With all due respect, liteweight, you are just plain wrong on this one. Tax law has nothing to do with yield at all - revenue have absolutely no interest in the "yield" and their documents and guides doesn't mention the term at all with respect to property investment. All they care about (for income tax purposes) is the income and allowable expenses. They simply don't give a damn about present (or past for that matter) valuations. CGT is a different matter but even then, "yield" is simply not used by revenue at all.
In purely financial terms you are confusing something that perhaps would be called "nominal rate of return" with yield. The latter is always calculated on the basis of current valuations. This is the case with bonds, shares (which pay dividends), commercial property and every other type of investment.