I decided to break the old chain at the half link. Although I'd cleaned and oiled it and it was ok in practice, it was stretched/worn beyond acceptable limits. I counted 93 links in the old chain.
Then I attached a fresh half link to the new chain. I'm replacing with a Wippermann Connex 100 1/2" x 1/8".
Here's a photo of the chainwheel (which as I mentioned earlier I'd straightened a tad). The cotter pins are new and I had applied just a little grease on their flat faces in a bid to prevent them seizing up in future. I tapped them in, rode it, tapped them in again, rode it and finally tapped in again and tightened the nuts. I'm happy that they are in properly now, but not stupidly tight.
The connector link is now in place. This is really so much easier than using a chain tool to re-connect. There are 93 links in the new chain too.
The outer plate goes on next.
The spring clip is then placed in position. With a screwdriver, just slide it so that it clips in place. NB: A reader, JJ, pointed out that the clip is the wrong way around. I think he's right! The closed side should be facing the driven side of the pedals - I must have been confused because the bike was upside down. However, it doesn't matter now, because since then, the bike was stolen from my student cousin to whom I loaned it.
The joined chain. Lovely! One thing I noticed is that since the wheel axle is now nutted a bit closer to the chainwheel (because the new chain is shorter than the old stretched one), the gear cable tension had to be re-set. It didn't take long, but I hadn't thought of that effect of the new chain.
Anyway, she rides so much better now. I could feel the increase in efficiency first from the serviced bottom bracket and then another incremental improvement with the new chain.