Must be more attentive mother
I gave DS a plum this afternoon, and then went to check my e-mails. When I came back it was ALL gone - including the seed! He was sitting exactly where I'd left him, didn't seem to have moved, so the only conclusion I can come to is he swollowed it! I didn't hear anything (I was about a metre & a half away) and he seems perfectly fine. I rang 13 HEALTH and they said see a doctor within 12 hours, and it was 6pm by that time so we took him to the doc, who said he may have a sit of a strain in the next day or so passing it and just to watch him.
I feel like such a bad mum - he's only had whole fruit in the last week and I've always been near and his given me the seed, this time I just wasn't being attentive.