Oh, dear! Blonde moment!
My son is sitting in his London office, laughing his socks off!
Apparently, I must have touched the cruise control button accidentally (I never use it) because it seems that LIM stands for 'speed limit set' rather than the more drastic 'limp mode'.
So he told me to turn it off with the button on the steering wheel, which I have now done.
I didn't dare tell him that in order to photograph the fault, I accidentally dropped my phone and it disappeared down an iPhone-sized hole behind the steering wheel! Luckily, I managed to poke it out again with my fingernail.
I think next week on shopping day, I might just catch a bus instead, I'm sure it would be a lot less fraught!