long term FTSE profits / strockmarket yields (ex inflation) over the tricky coming decades, at best guess, seem to be in the 6% ballpark, meybe a bit less
simple arbitrage says fair value for any asset (unless you have Warren Buffet like nose for future profits) should be based on an ex inflation yield of the same 6% - since long term property price datasets suggest that house prices (including in the UK) simply follow inflation over the long term, then this translates into a marginal imputed rental yields, after stripping out expenses etc, of 6% equates to fair value - ie taking your imputed rent minus your costs
and a yield above 6% indicates bargain territory, but do bear in mind to consider all costs