PROJECT UPDATE
This has worked out really well!
The replacement (not quite so worn) 15 tooth sprocket works well with the new 10 speed chain. I haven't had the chain slip again in the hundreds of kms ridden since the replacement. As the chain starts to wear it should fit the sprocket even better so I hope to get a few thousand kms out of the combination before it needs attention again.
I have only used the 23 tooth sprocket to help the chain up onto the 29 - I haven't actually ridden along in 42/23.
I have never used the 29 on any climb that I had previously managed in my old singlespeed gear. When I am forced to use the 42/29 gear on a tough climb, I always change back to my 42/15 gear as soon as I think I can cope with it, even when it will be hard work for the remainder of the ascent.
The lack of a gear lower than 42/29 makes for a clear delineation between my 2 road bikes... If I can ride a route on this bike without having to walk any of the climbs, or with a maximum of (say) 1 minute of walking, then this is the bike that I use. If I would need the triple chainset of my Cannondale, then that bike gets used instead.
If/when I run out of used 29 tooth sprockets, I would probably start replacing with 30s. I have a couple of those lying around somewhere and might as well take advantage of another 3.5% reduction in my bailout gear ratio.
I use the 14,13 and 12 tooth sprockets on long gradual descents when I want to be riding at 30+ kmph but I stick in the 42/15 up to 30 kmph and for short bursts up to 36 kmph.
I do many of my rides as if the bike were still a singlespeed. This is helped by the fact that the shifter that I fitted is bar-end AND on the left side of the handlebar, rather than the right; I often forget that I can change gears on the bike now!
The mirror that I fitted on the RHS of the bar is very reassuring. I don't use it to make decisions about turning, changing lane etc. but it lets me time looking over my shoulder better to avoid wobbling at critical moments. (Yes, I must start doing exercises for my stiff neck again - it is a safety issue, as well as being uncomfortable.)
The only potential issue now is that this bike has gradually gone from being a cheap DIY singlespeed shopping bike project made using mainly spare parts, to a nice 6-speed bike with some good new components added, which I would be very upset to lose if some lowlife nicked it while I was in a shop!