Who is Lloyds International Lotto Prize Inc.? ?

I recieved a letter stating I was the prize winner of 50,000 dollars in a lottery sweepstakes. I dont play the lottery....ever. It says my lot number drew lucky numbers. I called the number (london number) and talked to someone. 6-9 days later, i recieved a cashiers check for 4,450 dollars and was told to call and clear the rest of my funds. I was told to deposit the check and the funds would be available Monday. I would then call monday and get the rest of the funds. They did not ask for any money in return. The check looked good except signed with a stamp. Does any one no about this.