Mondeo immobiliser problem?

I have a 2001 2.0 Turbo Diesel Mondeo Ghia. It's been off the road for about 4 months as I don't really use it much now. Mileage is high so I have agreed to sell it to a friend for spares or whatever. Whenever it has been unused for a period of time before, charging the battery has always led to a first time start. (was once left for 6 months). However, this time, it seems to be encountering an immobiliser problem. Trying to start immediately takes it into what I would describe as sounding like a 'chugging' kind of start attempt. The engine is not turning over, and all the dash lights are flashing on and off. The red LED on the dash clock is off once I have opened it with the black key fob (the only one I have), but unlocking either manually or remotely still seems to make no difference. One possible idiosyncracy that might be relevant - when I was reconnecting the battery, the car had not been unlocked, so the horn/alarm immediately started to go off, and presumably would engage the immobiliser. Would that entail something such as disconnecting and then reconnecting the battery? I really don't want to get Ford involved if possible - it's only going for £150, so expense to get it started just to drive it to a mates would be a bit pointless! Hope someone can help. Much obliged