Graham,
You mentioned that you are using the avent teats with one hole, these are designed for newborn use and the avent website reccomends a medium flow teat with 3 holes for a 3 month old. I know that my 6 week old cannot get enough milk through the newborn teat and wriggles around in frustration as he is not satisfied.
I have attached a link to the website.
http://www.aventbaby.com/uk/products/catalogue/bottle_feeding/silicone_teats.html
You mentioned that you are using the avent teats with one hole, these are designed for newborn use and the avent website reccomends a medium flow teat with 3 holes for a 3 month old. I know that my 6 week old cannot get enough milk through the newborn teat and wriggles around in frustration as he is not satisfied.
I have attached a link to the website.
http://www.aventbaby.com/uk/products/catalogue/bottle_feeding/silicone_teats.html