Mortgaged home now too small, cant sell

HI All, can anyone offer some advice. My wife and I bought a house many years ago, we now have two children and our house is too small, (also our Youngest needs medical attention) which means we need a bigger house and one that caters for our youongest. We cannot sell or remortgage, so we have rented out our house and are now living in a rented home ourselfs. we need to inform the mortage company: are they likely to let us rent out our home, last time we asked we had to pay back one thrid of the mortageg and have savings, (if we had this we would sell up and buy elsewhere). If they say No, is there any legal loop hole. I have heard that due to the Economic climate a lot of people are having to do the same and some Mortage providers allow this. (it may be important to note that there is NO profit being made by renting out.

Any help[ would be appreciated, Thanks