I can also vouch for Sainsburys being very understanding. My mum worked for them part-time (after retirement age - she was bored!) for over 10 years. Unfortunately, several years ago she was diagnosed with Parkinson's. However, the branch at which she worked allowed her to continue working, within her limits, until she decided it was time to call it a day on her own terms. Her ex-colleagues still invite her out, although now my mum is often too unwell to take them up on the offer.
But I just wanted to add my bit to say that Sainsburys do seem to be one of the 'good guys' with things like this.