There is a lot of anti buy to let feeling on this thread. I am not a LL, nor likely to ever be one. However, I am aware that for many LLs, the decision as to whether to invest in a property, or in many of the other savings vehicles (PEPs, ISAs etc), is based on the return on the investment. They need not only to cover the mortgage, which may be only a small percentage of the value of the property, but also have a worthwhile return on their capital, compared to other investments and savings.
So to get a gross return on an investment of £500,000 of 5%, a rent of £25000 per annum would be charged, or just over £2k per month.
However, that isn't pure profit. Out of that the LL needs to pay for agency fees, management fees, repairs, maintenance, and insurance.
LLs are not just greedy capitalist bastards, (though some may be) but people investing their savings, or running a business and trying to make a living from it. If they are curbed too much, landlords will make the decision to stick their capital in a building society.
Then who will people rent from?