Thank you, everyone, for all your thoughtful responses! It is such a blessing to have this community and to have your support and advice.
I’ll utilise the calendar idea, and I love that very much.
You are also very right. I’m getting emotional now and letting the negative experience colour my perception of all the lovely sitters out there. Thank you for that reminder.
Sending you all a lot of love 
Oh dear, this really is unacceptable and should be mentioned in your review. It illustrates this sitter’s very poor standard of cleanliness and overall attention to detail. As sitters, we just don’t live like this and wouldn’t dream of leaving things in such a state!
Do you have hoses and sprinklers, or did the sitters have to use a watering can? Is watering the plants a vast job, or doable? Were the sitters frail, or was there any physical reason they might not have been able to keep up with watering?
If you went through the usual watering regime prior to confirming the sit and the sitters said they were happy to comply but then obviously didn’t, this is another issue to highlight in your review.
We once arrived at a summer sit to find there were over 150 potted plants outdoors which needed watering, daily (not mentioned in the listing). It wasn’t a problem because there were hoses, but it took a good deal of time every day so it really should have been included in the responsibilities section in the listing. We made sure we mentioned it in our review.
I’d suggest you take a day or two before writing your review to allow yourself time to calm down, then please do write a factual review, keeping things balanced and fair.
Thank you, @Happypets. These are all valuable points and a good lesson for the next time we need someone to take care of our house and cats.