If a lender has a bad credit rating the mortgage lender will be able to determine this from a credit report as soon as you apply for a mortgage.
Bad Credit Mortgages are still available from certain mortgage lenders most probably at a higher rate of interest and definately with premium mortgage insurance from a private company.
Poor credit ratings are caused by disliked financial practices such as deaulting on payments, making late payments, declaring bankruptcy or exceeding credit card limits. The best way to overcome a bad credit rating when applying for a mortgage is to let the mortgage lender know of your bad credit history and explain the circumstances leading to these problems rather than remaining silent.
There are mortgage companies that provide bad credit mortgages on a regular basis but it is always best to search for a company that specialises in bad credit mortgages which will increase the chances that you will get the mortgage you are after.
Every time you apply and are turned down for a mortgage it will be recorded on your permanent credit record and will only make it - that much harder for you to apply for any sort of bad credit mortgage in the future.