MPs, expenses, & mortgage payments?

hello., I have a genuine non-cynical question about the latest informaton on MPs' expenses. I genuinely do NOT understand why we, the taxpayers, pay MPs' mortgage payments on their second homes. Do they then keep their second homes, and the increasing value of those houses? So my understanding is; they have accommodation in their constituency, which (presumably) they pay for. Then they have a second address in London to enable them to visit the house of Commons regularly. Do they find the deposit? or does the taxpayer? We, the taxpayer, then subsidises them for the mortgage payment .... WHY? they then own this second house & can live in it, or sell it, at their choice. After we have paid for the property. I'm mystified. How on earth can this be justified or explainable?