Hi Hazel
My DD went off her milk about 9 months. I used to offer her an 8oz bottle morning and night, but sometimes she'd only take 4oz. I always made 8 oz up so the offer was there, and for the last couple of weeks, she's actually started to finish the bottle again. I think it's very common for them to lose interest, and the amount of food you're offering sounds fine(it's similar to my DD and she weighs 21lb). I am going to put DD on cows milk when she's 1, but will still offer the same amount so she's still getting the calcium. Also, we use Avent bottles and i have bought handles for them, so DD can feed herself, and she seems to prefer that, although i'll probably stop using a bottle soon, and put milk in her cup instead.
HTH
Amy