What colour is your sofa? If it's black or brown - try using the same colour shoe polish. As you use this to make scuffed shoes look new again - I'm sure this will work with your sofa too. Might be worth trying it around the back first though - just in case.
The trouble with shoe polish is that it could well mark your clothing when you sit or lean back on it. It may well be that your best bet would be to claim off your house contents insurance and replace it. It shouldn't make too much difference to your monthly premiums.
Go to your local boot menders and ask if they stock restorative colour leather cream or call into somewhere such as Land of Leather or John Lewis. You can get this stuff because I had the same problem with a green sofa. The other thing you can try (but be careful) is leather shoe dye.