"lie to let" is hardly news - if you do your homework you should know the values of the apartments that you are potentially purchasing and therefore already know what the true value is and what the true
true of the apartment is, and if you don't you should use an independent agent (ie. independent of the sale not just a resale broker!) and they should be able to let you know where the true value is.
As for the saturation of city centre markets - yes its true to some extent that smaller regional cities that piggybacked off the success of Manchester could be effected (such as Sheffield, Newcastle, Preston, Liverpool and to a lesser extent Leeds) but Manchester market is a lot more mature than the afore mentioned cities and there is reason Manchester was the first to boom and that is because it is a financial and educational centre for the north of England and has the ability ride booms and busts unlike smaller cities which could be more susceptible to economic variations.
I would still advise that a purchase of a second-hand city centre apartment or a house/apartment in a affluent southern suburb is a good bet for rental income and capital growth.