The buy to let sector is is only a small % of the overall market, but given that it is supposedly about investment and return I can see this dying off when interest rates rise next month. A lot a yields are now at 5% (monthly rental x 12 / property value) not including voids. If you can get 4.5% in the bank, why bother with the hassel of a buy to let.
The other issue is that where people are leveraging mortgages to increasing gearing, there are lending rules such that rental income should cover cover
130% of the mortgage. Basically when rates rise again this will immediately choke of new credit.
Buy to lets will therefore and in some cases already are generating zero or net income. People are buying purely on the prospect of future price rise, this is the definition of a bubble!