Thank you very much to everyone for taking the time to respond. There is no button on my car that I can find that stops the mirrors folding in - I bought the car from an importer who has quite a few of these vehicles in stock and they did identify this button on other models in different dashboard locations, including one exactly where Neville said it would be, but on my car there is no such button or at least I can't find it and neither can the importer.
I know that on some other Toyota models there is a function whereby you press and hold down the mirror fold button while pressing one of the arrows on the mirror position control button and this disables the automatic fold but I have tried this and it does not work.
I do know a couple of Japanese people who I see occasionally and my current plan is to take the original Japanese owner's handbook when I next see them and pester them to figure it out.
I am sure that getting an OBD reader is a good idea (thank you Craig) - I normally work on much older cars where they are not required, but I think on this vehicle it would be very wise to have one, I will look into the JPNZ manuals.