I got the car to start yesterday morning but it required some really fine adjustment to the position of the key in the barrel. I went straight to the dealers afterwards and they sent me down the road to their mechanic. He replaced the ignition switch but only because it's all he could think that it could be, rather than knowing that the fault was there. As a result we couldn't know whether it was fixed or not until this morning.
I went out to the car at 5:20am this morning and tried to start it, and it fired up first time. Phew, thank God for that! Whether it was faulty, needed reseating or the contacts cleaning, who knows, but the fault was definitely with the ignition switch, and it seems to be fixed now.