Thank you both for your reassuring comments.
It started when I noticed that LO would not look at us, even when we put ourselves in her line of vision she would turn away. Then I noticed she wasn't smiling, or fixing and following. She used to hold her head up for tummy time really well when only a couple of weeks old, but as she got older, she would just lay there. Her head circumference grew rapidly (this has retrospectively been shown to be a possible red flag) and crossed about 20 centiles ;o(. She wont turn to noise although I know she can hear because she passed her newborn hearing screen and she will startle to a sudden noise. She doesn't turn her head to me if I enter the room talking. And she seems quite a 'still' baby compared to the others in my antenatal group (I know you shouldn't compare, but impossible not to when you start to get worried).
Of these things, she does now fix and follow and WILL make eye contact, but only if I really make an effort to get her to look at me. Then she will smile. But the eye contact and smiling are still hard work, even at 20 weeks. My heart absolutely melts when I get the smile and I feel greedy and want more.
It just seems to me like every red flag going, I can relate it to my LO.