Just a post to move it on down the road.

I don't know Maggie. I think they twist, on their own accord. Perhaps the pot got moved and it twisted to get light?

Thanks for the reply, Cena, you're always the soul of help! It lives in the same window all the time, being brought inside the curtains in winter only if frost threatens. I do turn it often - I looked at it today and it certainly is very healthy. I was really wondering if all twisted Sans could be as a result of a virus. Probably there isn't an answer to that one.

Sans have 'layers' in their leaves...different colors that produce the 'variegates' for example...a true 'twisted' leaf comes from the different layers growing at different rates...If the offset reproduces plants that are also 'twisted', you may have a 'new' variety!
Good luck!

krimsonkreek, I've sent it to an expert, so I'll have some idea if it's unusual quite soon, hopefully.