Here is my (strange) update.
I went in to town this morning, the car locked and unlocked from the remote without any problems. Stopped at the local Tesco on my way home, again the car locked and unlocked from the remote without problem. Then I arrived home and parked - car won't lock from remote!! Changed the battery for good measure, if I hold the remote against the driver door window it'll work alright, no better than that.
Seems there is some interference after appearing from somewhere near my house?!