It had been raining and the door had swollen slightly. For a bit, I worried that I wasn't going to be able to get the door unlocked at all. The lock seemed stiff. However, eventually it relented and I was in the house. The door was, however, quite wet, and its wooden surface had some fibres coming off of it.
As the wet weather continued, it appeared that the door became wet through and this dampness brought some oil from the handle into the wood of the door. The idea of just putting a bit of clear varnish on it seemed to be increasingly unlikely. In fact, it took the act of driving away from B&Q with some clear varnish having been bought for me to reach this conclusion. Idiot!
So, on Saturday, after a later wake-up than planned, I drove to another DIY shop, got them to help me design my kitchen, and then bought some quick-drying teak-tinted varnish. On Friday night I had spent a fair amount of time sanding the surface of the door, running through three grades of sandpaper. Eventually, returning home with about 3 hours to do my varnishing in, I got something weatherproof on the outside of the door, and most of the inside.
I don't really like dark brown wood doors, but the staining, which was deeper than just the bit of surface I sanded off, needed hiding. However, the door was looking reasonable. There are 9 panels of glass in the door, so the effort of painting the glass retaining bits of the frame with varnish is the majority of the work. As a result, I didn't do the interior of the door in varnish around the glass as there wasn't time.
For weatherproofing it requires 3 coats of varnish, though the 30 minute drying time to rain-proof, gave me some confidence that I'd improved the door's lifetime by about 100 fold after the first coat. I'd received a note from my builder saying that if I didn't treat the door soon it would disintegrate. There was more to do to it, but it was, at least, going in the right direction.