Door sensor problems

Please can somebody tell me why my car thinks the drivers side door is constantly open on my p38 4.6 dash.First time it started [about 2weeks ago] the message came and went off, but the alarm would sound when I locked the car sometimes often, sometimes only once.Now the alarm seems to have got bored and dos,ent go off at all.but the message on the dash is constantly on.Today to cap it all it read gearbox fault but this has not shown since.I have tried lubricating the door lock with wd 40 but without success,these faults have all started since the cold spell could this be related? otherwise the p38 has been well maintained.

strip the door mech out and clean it up including the connections.

Cheers Gav,I will put the beast to the knife as soon as this snow disappears.[great to watch other cars in this weather though]

After atempting to remove my door lock mechanism i discovered land rover made it almost impossible to remove the door handle to main latch rod there seems to be a clip at the top end,can anyone tell me how to take it off