Staying at Kate's place was the best decision we ever made. Her place is very lovely and so is the family. Kate and Steve are great host. Kate will respond to your emails very fast.
I would recommend you to stay there. The location is the best. It is just 15 minutes walk to the Primrose hill and the view of London there is just amazing. The shops and the tube are just 10 minutes walk away. It is the northern line so you will get to the center of London very fast.
The neighborhood is super calm and safe.
And next time I'm in London I will definitely stay at Kate's place.