Hey, just putting it out there, but if people can afford to acquire a second property and are using the rent from it to pay a mortgage on their own home or increase profits/create a nest-egg, it seems unreasonable to pass on mortgage inflation to tenants, who only have one temporary home. It's not their fault mortgages are going up - at least split the difference with them!
A second property, whichever way you look at it, is a middle-class "luxury" and should demand fair rent prices and promote less greed. Buy-to-let is a risky business, but the landlord takes the risk and should not expect to put up rental prices on a whim or to charge extortionate prices for no service improvement (as per most business models).
I speak as a second property owner, who charges their tenant only 2/3 the geographic expectation because the tenant is on benefits. She looks after the house beautifully for a 1/3 reduction and her (local government-generated) rent is reliable. Any income at all is a lot more than most people have right now, and if you can't sustain a second mortgage comfortably without burgeoning your tenants with your costs, surrender it and put your money into something else.