Bad missfire

[Ged]

Went to take my R300 Duratec out for the first time since last year, but after a few hundred yards a missfire started and got progressively worse. Got home and pulled out the plugs, they're all really black and sooted. I changed the oil and ran it up to temp on the drive a couple of weeks ago, it seemed to run fine but obviously not under load. It's still got last years fuel in it, but never had a problem before. Anyone got any suggestions of where to start. Could the Lambda probe cause this? My MOT tester suggested it might be on it's way out last year.

[SM25T]

Disconnect the plug on the lambda sensor and start the engine. If it drives normally, replace the sensor. Remove it and read the engraved number on the body. Find a supplier on line.

[Ged]

I've changed the plugs for an old, but good set and changed the Lambda. I'd already bought one and it seems to be driving fine. I'll pull the plugs out again later to check them and replace with new tomorrow if all looks well. Thanks for your reply, it looks like the Lambda was the correct diagnosis.