Inspired by the comments here, I had a go yesterday, nothing ventured, nothing gained. Honestly, I don't think I would have had a go without hearing that so many of you have had to grit your teeth and go for it.
I stayed reasonably well back from the mike, so I could hear myself rather than the PA, I forgot my earplugs (we don't practice at great volume) 🙂
To cut along story short, Dakota was fine (not surprising!), chorus of Rebel Rebel was impossible, I managed the 'Please.... Please' on Stuck in the Middle with you, and was pleasantly surprised to cope with the tail end of Baggy Trousers, although the chorus was too much going on!
No-one complained, which means either they couldn't hear me or I was more or less in time/tune. Yes, I did check my mike was live 🙂
I've got a reasonable range, but when I can't hear myself well I tend to jump up an octave into my head voice at inappropriate moments; this is something for practice, but less important for backing vocals as they tend not to go up and down a lot.
So I shall practice at home, stick to the ones I can sing without fumble-fingers and gradually approach the mic closer until I reach the point where I get asked to keep it down 🤣
Thanks folks, I hope your suggestions encourage other people as well.