Thanks. I've got John at Spa on the case.
I've just checked their site and it does look like 41T middle rings do not exist so that won't be an option. If they don't have them I don't think I'll find them anywhere.
I think keeping that 11T gap will be the most important thing so would need to either go up to 53 or down to 51. 53 would be ideal for TTs but means I am not likely to get less than a 29T inner. I see they don't exist either, so either a 30 or, if I am very lucky, 28.
28/42/53 would be ideal, if it will work. 30/42/53 more likely to work but slightly higher bottom than we currently have.
The other option is to just go for a double, 11-speed and an 11-40 cassette with GRX rear mech. As I see that the options for triples are quite limited, I'm gradually re-realising why that is what I have on my own bike! Only downside is some large gaps between the gears but I think we can both live with that.