Success!!!! As I walked into the flat there was such a delicious smell of chicken cooking. It had been on for about four hours and the chicken and vegetables were perfectly cooked.
The vegetables had created quite a lot of liquid, so I didn't need to have added so much water. But I like my food fairly dry, so that's something everyone would have to decide for themselves.
The chicken was still above the liquid, so it had steamed, really. It was falling apart and absolutely lovely. Obviously, larger pieces of chicken, or a whole one, would need longer.
The butternut squash was very soft but not mushy. The leeks were slightly overcooked and I would cut them into larger chunks next time.
The only flavourings I used were salt and pepper and this allowed the taste of the meat and vegetables to shine through. I have eaten half of it, and might have second helpings later, or I might save the rest for tomorrow. I will drain off the stock and chill it, remove any fat from the top, and use it for something else.
(The outside of the cooker gets quite hot - it is stainless steel - which I hadn't expected - silly really.)
I am absolutely delighted with the results, and will definitely be trying out other recipes in my slow cooker!