Buy to let mortgage for 90% is becoming more popular with the current demise of the 100% buy to let mortgage.  Why is this?  basically the buy to let marketplace has had a lot of bad credit mortgages written, and this has lead to the mortgage lenders being much more careful about the amount of debt they are willing to lend, and the percentage of debt being used to purchase a buy to let mortgage of over 90%. 

This 10% difference gives a margin of error upon house valuations and the property market dropping, so the lender has less risk to the turbulence of the buy to let mortgage market, since they in effect have a 10% buffer built into the buy to let mortgage 90.