Diagnostics will tell you exactly which lambda sensor is faulty (bank 1, sensor 1, etc), and whether it's the sensor itself, or the heating element. In any case it can sometimes be a wiring issue, so need to check that also.
If you fix the issue then the MSCF light will probably go out by itself after several starts. But it's so easy to clear codes why wouldn't you? If you have a smart phone, download the free torque app and you can use that. Just need a ELM 327 Bluetooth device from ebay or wherever. Costs a few pounds.
If the garage haven't already, you should clear the code first and see if it comes back. Sometimes they can just be a glitch and there is no need to fix anything.