From what I understand, with buy to let mortgages, as long as you can put up a 25-30% deposit, the lenders are only concerned with the rental income covering the repayments. Many people who have rental portfolios are only paying back the interest on the loans and using the difference between rental money and repayments as income. The theory with this is, long term the value of the property will increase meaning that you it can be sold and the mortgage paid off, leaving a nice little sum. It has to be remembered that this profit will be subject to capital gains tax though.
This is all well and good providing that everything goes to plan. It's all very well being mortgaged up to the hilt on a number of properties, but at some point that money will have to be paid back. You only need a few months with a property being empty and it could all go very wrong. It's a risk you take I guess, but those that have it, didn't get it without taking a few risks along the way.